I recently acquired a nearly mint '96 Big Foot C2500 (the older 2500s are almost identical to today's 1500s) with the center-left kitchen w/ the wet bath rear-left bath. Mine doesn't have the basement as do the newer design C2500s, for instance.
There's a small door on the forward bath wall at the bottom with a 2" blade flush valve behind it.
Here's a photo link: 2014-11-17_2015 - bobinyelm's library
I suspect it closes off the shower drain to the gray tank so it can't back-slosh into the shower floor, but the BigFoot owner's manual (unfortunately it applies to the TT, 5th, and TC) so doesn't mention the valve.
It also didn't mention the location of the HWH bypass valves (which I located by removing a cabinet drawer), so I figured it was better to ask here.
Also, there/s a heavy gauge (#10 or 12) yellow wire not part of the wiring harness in the bad-step that makes a U-Turn, and has a 20 amp inline fuse in it. It isn't labeled to say what it controls, but it sure looks factory.
The right rear dinette makes into a single bed, but with the table set into the supporting ledge-boards, the rear aisle part of the table is completely unsupported. Is there a portable leg or something that is supposed to fit under the unsupported corner to hold it up? Without this, any weight placed on the un-supprted corner would cause the table-top to flip off.