Appreciate your thoughts, everyone.
The RV right now is on the well water that my house is on -- no difference between the RV and the house how it is treated.
In the house the water can sit in a toilet after flushing and cleaning for 3 weeks and it is just fine when I get home again. It only needs 8 hours in the RV toilet and it's clearly changed colour and odor.
The RV toilet is clean -- first thing I did. Even did it again to be sure :-)
I'm gonna turn the water off in the RV, empty the water from the toilet bowl, then pour RO water in there from our kitchen sink in the house. And I'll see what happens to that overnight. I'm thinking if it's OK in the morning I likely need to sanitize my RV water system. If it's not OK then there has to be something going on in the toilet itself methinks. I'll keep you all posted. Appreciate what each one has suggested.