I know that most people covet a dry bath, but we have had a dry bath for the last 23 years and can say that with the limited size of the GMC it is not a very good option for big people. I am 6'5" and my wife is 5' 10" and we found that the dry bath in the Royale was just too small for comfort and that the shower was so small that it rarely got used. In fact I would guess that in the 23 years of ownership of this GMC it was not used a dozen times. We either stayed in campgrounds with showers every other day or so, or visited our friends with big SOBs and begged a shower from them: though after several days without a shower begging was not necessary, they provide the shower in self defense.
The following image shows the original bath.
After looking at the options it appeared that the space used by the shower could better be used in making the bath larger and adding some badly needed storage to the kitchen. Thus started a fairly major remodel of the GMC interior.
The first step was to strip out the interior from the cockpit back to the closets except for the kitchen and re-insulate, put in a new headliner and new wall panels. The following picture shows the bath area after the shower and bathroom cabinet are removed but with the toilet still in position with the fixture to raise it several inches. Even the tile floor that was installed last year will have to go. It was amazing how little storage was in the original cabinets after the original manufacturer installed the duct work and plumbing, They just did not seem to know how critical good clean storage is in a vehicle this size.
The design for the bath was somewhat dictated by our accidental acquisition of a bath counter top while visiting Northwest RV Supply in Eugene Oregon. They sell surplus RV supplies they have gotten from major RV manufacturers in the area. This was a L shaped solid surface counter with sink at a cost of $100.
Using the computer to create initial drawings of a new layout and then cardboard to build several mockups we came up with a design that we thought would work. The resulting design used about half of the original shower in the new bath and left the rest for use as pantry pullout storage in the kitchen. Had we known how much added storage we would be able to create with the kitchen remodel that followed we would have used more of the space in the bath and made the pantry pull outs smaller. The drawing blow shows the original configuration in gray lines with the new configuration overlayed in black. The yellow shows the storage areas allocated in the new bath.
The toilet location is not very flexible since it needs to be plumbed directly into the black water tank so it ended up only 1.5 inches outboard from its original location. The cabinet work ended up being totally different with an incredible amount of storage for a small bath.
We also wanted to have shower possibility so opted for a wet bath design.
Some complications did appear. Due to our height we needed to keep the floor of the bath at the level of the regular GMC floor. This did not allow the shower to drain into either of the tanks with the required trap. To solve this problem I chose to go with a marine industry solution and added a shower drain pump to move the shower water to the holding tank. This has a significant advantage of being able to now put the shower water into whichever tank has the most room, and we have also provided for a direct dump to the ground if we are in an appropriate location thus saving space in the holding tanks.
The outside walls of the bath were insulated with additional foil back foam board, with 1/2" on the upper walls and a full inch to 1.5 inches around the wheel wells.
The cabinet work was built with 1/8" and 1/4" plywood with cedar framing to keep weight to a minimum.
The shower drain location was dictated by the location of the black water tank, the gas tank and frame. It ended up going through the floor between the black water tank and a frame cross member. I had to be a little creative with the plumbing underneath to connect up to the pump, but it all seems to work.
A neat benefit of doing your own work is that it is possible to do things like raise the toilet several inches to make it more comfortable for us of the aging generation. We had already replaced the toilet with the regular style Sea-Land china bowl unit and added a riser but still felt that additional height would be more comfortable for us tall people so it is now even higher.
Here are some pictures of the bath after initial construction of cabinets. note the amount of storage that can be had under the sink by just being careful with the routing of ducts and plumbing.
To make this a wet bath I built a fiberglass pan in place and then fiberglassed all the walls and cabinet work. For the pan I pre-sloped the floor to the drain by troweling in a layer of resin mixed with micro balloons but could have used almost anything from resin with sawdust to thin-set mortar. After the floor was properly sloped at about 1/4" to the foot I then glassed over it using fiberglass matt and cloth and polyester resin. This went up the sides of the bath about 4 inches all the way around. This required adding a 4" threshold where the door was located. Glass cloth and mat was added until I had about a 1/4" layer. which will be strong enough to stand up to any king of abuse.
For the upper portions of the bath and cabinets I first rounded all outside corners and then filleted the inside corners with a mixture of epoxy and micro balloons. Here the use of the micro balloons really helps since they smooth very nicely and required very little sanding before proceeding. I then put on a layer of 4 oz cloth with epoxy over all the walls being sure to overlap all corners with two layers and let it overlap the fiberglass pan on the bottom so there would be no joints to leak. By using a minimal amount of resin (just enough to wet out the cloth there is very little sanding to be done. If a second coat of resin is put on as soon as the first coat gels then no sanding is required between coats. If you are careful only minimal sanding is required after the second coat and then mostly at the seams. I used epoxy for the walls mostly because there are a lot less fumes, but even then I used a respirator for all the work and an exhaust fan to remove any fumes.
I then sanded the interior and painted with a two part off white epoxy which I had on hand, otherwise I would have used a mono-epoxy or urethane enamel.
The cabinet doors were built out of 1/2 inch Baltic birch plywood and painted with 2 coats of primer then sanded and painted with the epoxy. This gave almost a glass like finish.
Since this is a wet bath much thought went into how to seal the doors. I came up with many schemes, most of which required fancy door configurations or special work around the door openings or expensive moldings or seals. One day while at Home Depot I was looking at weather stripping and found a cheap stick on weather seal that works great. I used touch latches behind each of the doors to hold them closed so the doors are smooth and clean looking.
The L shaped counter top from the salvage yard was then cut and modified and placed on the cabinet and sealed around the edges where it meets the walls.
The faucets were pretty much standard Delta products from Home Depot. For the shower we are using a sink sprayer on a long flexible hose which tends to keep water usage to a minimum.
The floor pan was tiled with 2x2 porcelain tiles from "Daltile" which are very thin and therefore very light.
The following pictures show the bath after completion. Note the amount of storage, including a large cloths hamper which has a net bag the hangs inside and opens when the door is opened.
A shower curtain is used to cover the entry door while taking a shower.
Since finishing this bath and spending 6 weeks in the Arizona desert this winter we used the shower in the wet bath format many more times than in the previous 23 years of motorhoming with the separate shower stall.
Hope this gave you some ideas to work with on your own projects.