Here's the proposed wash / black water system for "The Bat Cave Express". It will allow flushing the black tank with the wash water, pumping out the black with either pump, pumping out the wash directly.
I such hope I find a place not under the trailer. The Tanks are inside the wheel wells under twin beds. The Y valve's will be under a raised platform (3")in the back which the head will sit on to bring it up to a better height. The handles will be above the deck. The Grey Tank knife valve operates from the inside aisle. The two dump valves are the hardest to place. Looked at the electric operated ones but they are big $$$. Hope to come up with a better placement for the dump valves and outlet. The outlet itself is a little challenge. I want a pull out 1.5" hose but haven't figured out how to do it yet. The major point of this design is the options of moving water around and the redundancy. Running both pumps I'll empty two 55 gallons tanks in 5 minutes in a dual direct dump or 9 minutes running the Grey into the Black and out.