When the shop vac sucks up pieces of the subfloor, it's a sign.
I took the hint and decided to have a closer look at that section of floor adjacent to the back bumper. The floor was just rotted... I knew that when it rained it got damp back there but I didn't know I could stick my finger through it.
Got the bathroom somewhat dismantled since it seems like a good time to diagnose the leak when the toilet flushes....
Got the toilet removed and dumped some nice clean water in the tank through the big hole in the top. Expected the leak was around the toilet flange but WRONG - ran outside to see the water running out on the ground from between the panels in the belly pan sections.
So, now preparing to drop and fix the black tank, figure out where the impossible leak is that made my floor into mush, and probably get the froze/sploded water heater tank replaced, and install the new 12V
converter/charger that I picked up from VTS a while back.
Thanks to Shari and all the rest of you who have provided inspiration to tackle this project!