Hope this will help.
Sounds like the toilet paper or some other items may be stopping the flow. Do you use chemicals in the take?
This will not be a pretty picture. It could range from a faulty valve to a heavy concentration of paper, or 'other items'.
My suggestion is to get a "sewer snake" as I call them, and run it down the toilet side to see what you get when you pull it out. Then I would open the valve, hopefully the tank isn't full, and run the snake up from that side and see what you get there. Strongly suggest you get a large plastic container to catch any out flow, if you know what mean. Hopefully there isn't a build up of solids causing the backup thus slow flow out. If so it will take some time to get it all out with the snake. Use lots of water.
Did this on my friends motor home and found clogged paper was the issue. It only took several hours to clean it out. His wife had been putting paper towels into the toilet.
There are RV repair p;aces which have a unit to do the and a TV/Monitor which wil let you look inside the tank, but this will cost you some $$$.
Interested to how this comes out, no pun intended.