Thanks for the suggestions, Jack. When we realized we'd be doing some "hands on" work in a place where we weren't crazy about putting our hands (even when wearing gloves), I did a double-duty cleaning job on that toilet bowl. Nice to know my work was appreciated!
We're hoping the problem is with the seal rather than with the flush ball. The seal replacement seems a bit more straightforward and an easier repair. Still, I have visions of us rolling into 7 Feathers continuing to deal with a plastic bag filled with water being used as a temporary seal while we're scratching our heads and wondering what toilet repair we need to try next. It's just so weird that it worked perfectly prior to emptying the black tank and then the leak developed. It just has to be the seal . . . something got into the seal or the seal was damaged when we cleaned the bowl . . . who knows?
Carolyn