Tank draining issue
I use a Flush King when dumping the tanks. There are other brands that do the same thing. It is a clear device with a gate valve and an upstream hose port. When the drainage is done, you close the gate, hook a garden hose to the smaller port and back flush the tank. It takes a few minutes to accumulate the fresh water, then close off the fresh water supply and open the gate valve. You would be surprised what will come out. You are flushing the whole drain system, from the outside. I flush the gray water tanks too, to be sure that the valves are clear. 2 or 3 times usually clears the tanks.
Another comment about the 'concoction'.... be sure that any of the ingredients do not contain chlorine. Ammonia and chlorine can be very dangerous, it mixes to form a gas that is toxic.
One last thing, when we had a 25E, the drains were not pitched very well, I would put a plank or two to raise the opposite side to help with the flow. Our Jay Flight does not have that problem.
Good luck with the drains, many good suggestions.