[Wally Arntzen]

It sounds to me like both of the gate valves are probably bad and need to be replaced. I did both of mine and now have a set up that can be removed with removing to stainless clamps that hold the entire unit together. If you have a problem with sewage messes get a plumber and it can be done in about 30 minutes.

Cut the line near the tank leaving enough room to reconnect the PVC pipe. There will be a mess but you can hose out the entire compartment to clean it up.
Trying to repair it without a mess is almost impossible. If you cut the line you can put a preasure washer tube into the tank to bust up any build up you may have on the bottom of the tank.
Chemicals are not going to correct the problem you have with the defective gate valves and I would try and drain what ever you can out of the cheater dump on the drivers side of the coach. The valve on that side is probably also in questionable conditiion and will need to be replaced.

It's like eaten barbeque ribs you gotta get into it and get a little messy and if you can't, call a plumber who will jump on it like a fly on a terd.
Good luck.