Thanks for the info. I'll have to check the fuse box again, but last I checked there was only one wire that looked like it was installed afterward into the fused hot terminal - it was for the CB.
Problem is there are no other wires down there anywhere. I really wish I had made note of where that fused wire goes. I'm 99% sure it was on that sender like you said, but there was nothing else hooked up to it. There may have been a fuel pump installed at some point but there's nothing there now. Unless it's in the tank, which is not likely. I have the rubber feed disconnected at the tank. It turns to steel after about a foot and is uninterrupted the whole way to the engine. Curiously, there's a ground bolted to the tank: it's a yellow wire like the one up by the oil pressure sender. But that would be silly - sender to ground. That wire is covered in foam so it's original from either GM or the RV builder.
I hooked that big black wire up to the oil pressure sender (the one for the gauge) and it didn't seem to do anything.
In any event, I'll be dropping the tank this weekend so I can see what's up there. I've drained 55 gallons out of it so far and hope to get the rest out today.