Interestingly, I just noticed that my FRESH water tank overflow tube was dripping pretty substantially. I checked the inside monitor and the fresh tank shows full, which I'm pretty darned sure it wasn't when we arrived three days ago. Ok, we know there's a valve in the plumbing compartment which, when opened, directs fresh water into the fresh water tank. If that valve was leaking, allowing water into the fresh tank until it overflowed......how could THAT water get into the toilet and black tank?
Aaaaargh!!!