I've completed all the repairs to my sewer system. You will recall that I had a leaky gray tank valve, and a broken black tank.
The leaking valve was the result of a misplaced seal. It had come out of it's seat. To complete the repair I disassembled a valve designed for attaching to the sewer outlet and replaced the old parts in my gray tank valve with them. The parts were identical and I had no problem putting them in place.
The black tank was a little more difficult. I spent considerable time flushing it before disconnecting anything. I used both the flushing system installed at the factory and a flexible wand with the spinning nozzles. It worked quite well and I was happy to see that I had no residual waste in the tank to deal with. The break in my tank was along the top edge and side and was about 30 inches long. I had attempted to repair this once before using marine epoxy but this didn't hold.
I used a sander with 80 grit paper to remove most of the old epoxy. It actually popped off the plastic tank very easily. I also use a dremel tool and a chisel for getting the old epoxy from inside the crack itself. To re-seal the tank I used a combination of black ABS cement and fiber glass drywall tape. I put a base coat of the cement inside the crack and on top of the tank. While it was still tacky I placed a strip of drywall tape into the cement and over the crack area. After letting it dry a few minutes I applied more cement and two more layers of drywall tape. These were offset to the sides so that they covered the crack with the edges and so that they overlapped one another and the preceding layer. I let these dry as well then place one more strip of drywall tape on top of the previous three layers and used the last of my cement. This made an excellent patch that is about 34 inches long and 3 1/2 to four inches wide. I let everything dry until the cement didn't feel soft or tacky.
I'm pretty sure this patch is actually stronger than the original tank was and I am totally confident in the overall repair.
The black tank originally broke when after emptying all my tanks, I was temporarily distracted from what I was doing for a short time. When I came back to the project, I turned my flush system on when I thought I was turning it off. Since I had the valve closed the tank burst when it couldn't hold the pressure from the hose. The gray tank valve failed because the bolts holding it together had not been tightened properly at the factory. When the tank was totally filled the pressure forced one of the seals partially out of place and into the sewer pipe. This caused it to leak around the handle hole and was difficult to locate.
Here are a few of my rules for emptying tanks:
1. Never, Never, Never try to multitask when emptying sewer tanks. Stay with the dump valves and pay attention to what you are doing at all times.
2. After completing the job, NEVER leave a water hose connected to the sewer flush connection and a faucet. I once overfilled my tank when I found that my plastic shutoff tee's had tiny leaks that slowly filled my tank. A gurgle in the toilet when the tank overfilled raised my attention and made me check things out. By disconnecting the hose you remove the risk.
3. If someone stops to visit, stop everything and conduct your visit. When the visitor leaves, you can safely resume your project without risking damaging anything.
4. Double and triple check if necessary to be sure all your valves are in the proper position. If boon docking, everything is closed. When in a park using a full hookup, keep the black tank closed and open all the other valves.