Marine toilets are problematic, I’ll say. I am NOT in the mood to deal with this!
[Update! (20:00 UTC)]
Hallelujah! A gallon of Clorox poured down the toilet’s gullet (after dumping the holding tank) and an hour’s wait (dis)solved the problem. A complete water refill of the tank, and another dump, and whoosh! PROBLEM GONE!
I really expected this to be much uglier than it turned out. Part of the good fortune is probably related to my use of RV/Marine toilet paper. So, the Clorox reacted much better than under other circumstances.