OK, it took me 10 years, but I finally did it! Thought my grey tank valve was open when it wasn't... Boy did I get lucky. My shower only has a small tub, but it was full - and somewhere it was leaking just drops to outside, which is how it got discovered (by one of my neighbors noticing a drip).
If any of you every ask about what to do when temps get cold and I say I don't recommend dripping your faucet - well, that's how this filled - overnight! It's something I don't do --- just in case -- and here I am an example of what that just in case can be. But no water in the rig, cabinets, floor - so again, I'm still so very, very lucky!