There is a couple restoring an old sailboat on YouTube. It had no black water tank. Instead of trying to plumb one in, they built their own composting toilet.
They have a special "funnel" (for lack of a better term) that catches the urine (guys have to sit down) and send it off to a container while the solids go it a small trash can with a plastic bag liner. Coconut husks are added on top after each use.
The key is a tight fitting lid that seals the compartment under the seat and a small fan that that runs all the time sucking air out of that compartment. After a couple of days, you change bags.
I would not want to try this on an RV, but a composting toilet (option)makes a lot of sense on a very small TT.