Never planned on going inside the underbelly. That would have been a whole lot of extra work, which I have an aversion to!!! Also, the slide racks would have been in the way probably. Mod took about 1.5 hrs, 3 cuts on the orig pipes, and a small amount of ABS cement. No rivets were harmed during filming of this event!!!!!
Yes, the valves are inside, and as you can see the tanks feed outside the belly after a good drop. Inside probably would have had poorer drain slope.
As to the debris issue, or standing water, I do have a hose cap on the 3" cap that can be used to drain any remaining water when in driveway, and trailer is in a very down nose attitude till finished leveling. I generally do all flushing at home, and down the toilet. Less time in the dump stations!!!