There is very little original content in this page. Most of what has been done here has been done by others and it is through the GMC NET that I gathered the necessary information to repeat the process on my coach, Many GMC NET members provided information and ideas that were combined into my final installation. Many GMC NET members provide quick answers when I had questions or problems. Thanks GMC NET.
Even though I knew our tanks had some leaks on top I had been putting off a replacement until I got around to remodeling the Bath as I was sure there would be some changes in plumbing locations. 2003 proved to be the year to do it all.
Our 1978 Coach is a Royal with Side Bath and center kitchen. This coach has 2 tanks, a large black water tank between the bogies and a smaller gray water tank aft of the bogies which served only the kitchen. Coachman (who Built the Interior of this coach) must have thought shit flowed up hill since it was impossible to dump the holding tanks without putting the back of the coach down as far as possible, and if you really wanted empty tanks you needed to get the front wheels another 6 to 10 inches off the ground. Many people must have wondered what was going on when in the dump process I would get on the back bumper and jump up and down to get things flowing. We then had the long wait while the electro level system pumped the coach back to ride height.
Because of the dumping problems and a bath remodel; replacing the tanks in the original configuration was not a good idea so we opted for a better layout with a macerator and retaining emergency dump capability through a standard 3" sewer hose.
The tanks were purchased from Duane Simmons as the standard replacements for the Royal configuration. We purchased the GMC-2 and GMC-3 models. Duane has the fittings placed in the tank wherever you want them which made this project possible. I was able to even order some additional 3/4" fittings in the sides to allow hooking up a flush system. Button sensors were also included for detecting the level of fluids in the tanks.
A note of caution to anyone ordering the large GMC-3 tank for direct replacement in their coach. This tank has the exact outside dimensions of the original tank including the flange. However herein lies a problem, since there is no flange the tank is actually bigger and will not fit as far to the driver's side of the vehicle as the original tank due to a bogie frame member. This pushes the outlet more then an inch to the passenger side and required making the opening through the frame for the outlet of the tank larger. It also puts the tank much closer to the exhaust pipe, close enough that I felt a shield was necessary. Before ordering a tank I made a mockup out of cardboard using Duane's dimensioned drawings and positioned it in place and then marked the locations of the fittings on it. The mockup of the tank was three dimensional on the drivers side so I could see how close I could get it into the frame.
I chose a tank configuration based on Duane's recommendations where the tank outlets face each other and are tied together with a 3" 4 way fitting. To allow me to keep the gray and black water separated and have the ability to dump which ever tank I desired I included a standard slide valves on the outlet of each tank and a third valve for the backup 3" dump.
The following diagram shows the basic configuration.
When removing the old tanks I found that the 1 1/2 inch drain and vent pipes were permanently glued into the tanks and had to be cut out. I did the cutting above the floor inside the coach to remove the tanks. The new tanks come with a threaded fitting.
Also since my tanks had the button type sensors in the tank sides I took pictures of the wiring so that I would have a record of the color codes for connecting up to the new tank. Digital Cameras are wonderful at recording how things are before taking stuff apart.
While the tanks were out I took the opportunity to clean up the wiring that runs along the Drivers side frame rail and routed the Propane tank gauge wiring so it did not go over the top of a tank as it did in the original configuration. All the wiring was placed in the black corrugated wire loom.
The next photo is looking at the gray water tank on the ground from front to back with all the pluming connected to it with the exception of the black water slide valve which is already attached to the black water tank. The 3" elbow on the right is rubber and along with a short section of pipe and standard coupling provides the emergency dump. It is held up above the frame with a shock cord and there is enough flex in the rubber elbow to allow it to drop down to connect a 3" sewer dump hose.
I used flexible rubber couplings to attach the 1 1/2 inch drains and vents into the tanks. This was done by screwing a short plastic nipple into the threaded fittings that were provided on top of the tank and attaching the rubber coupling before raising the tank into position. The next photo shows where a rubber elbow coupling was used to connect the sink drain into the gray water tank. A straight coupling was used for the sink and vent into the black water tank.
The next photo is looking forward under the coach at the black water tank in position with the slide valve and the rubber coupler attached. The gray water tank with all plumbing attached will be lifted into position and attached to this tank. If you look close at this photo at the left front edge of the tank you will see that it is about 1/4 inch away from the frame, and notice that the 3" outlet is several inches to the right of the old position and the hole through the frame has been enlarged. This photo also shows one of the three 12 gauge galvanized steel straps that holds the tank in position. The bar that strengthens the frame by bridging across the bottom has not yet been installed.
This next image shows the Starboard side of the tank and the exhaust pipe. As you can see there is very little clearance. Before we left on our trip I installed a heat shield out of 16 gauge galvanized steal
The final install has all the slide valve handles extended to the outside of the frame for easy access and a skid plate that protects the back tank from being drug on the ground.
The outlet from the macerator is 1 inch spa hose. It has a smooth boor on the inside and a ribbed wall on the outside. This hose is a little large to slide down a conduit under the coach so I role it up and place it between the the Onan and the house battery. I found a plastic plug in my sailing parts box to put in the end of the hose to be sure that no unsavory contents left in the hose get discharged in the Onan compartment. The one inch hose allows for a slightly faster discharge then a 3/4 inch garden hose but probably not enough to worry about.
What are my impressions of a macerator? - All those great things people have said about macerators are true. After my first outing I was thoroughly convinced that this is the only civilized way to deal with RV waste water. It makes dumping a clean fast operation and I don't need to look for the dump without a curb and try to find one that slopes in the right direction. As yet I have not even made use of some of the great options for dumping in unusual locations and still think it is the greatest thing since sliced bread.