Judgerr,
Your toilet is just fine.
That is not your problem.
What is your problem is one of three things sort of in order here:
**Your black tank dump is plugged up.
**Your black tank vent is plugged up.
**You have a massive "black pyramid to deal with.
Start with the first - If the dump is plugged up with a mix of paper and solids, when you pull the valve, you may get just a trickle. Get a flashlight. With the lid down, step on the pedal. Now, with the lid up step on the pedal and if you see something other than tank bottom. We have a good start. You are at Nr.1 or 3.
Nr. 3 does not usually cause a fountain effect.
Fixes for a Nr.1 case:
First and easiest is get some enzyme based digester and put in a bunch and hope and wait. If next time you pull the valve it dumps at all, you scored. Now repeat this until it is really boring.
Second and not as clean, get the dump cap with a garden hose connection, a male to male hose connection, a hose thread valve and a backflow preventer (This is an ESSENTIAL component). Now put that all together so you can put pressure in the dump pipe***. Put some pressure in the dump pipe and pull the valve open. allow 15~30 seconds and close the dump valve. Take the cap off and be ready to catch what comes out. It will not be clean. Now attach the dump hose and try again. This can easily take three reps to get good results.
*** This technique is very hazardous because the dump plumbing was not installed with any expectation of pressure in the system.***
You can also try to run a plumbing snake up the dump pipe, but I have never seen that less than a disaster even thought it did clear the problem. When the dam gets breached... Katy bar the Door!!
Fix for the second case:
Get a plumbers snake and get up on the roof, take the cap off the vent and use the snake to clear the vent. The last foot may come up nasty, so be ready.
A Fast fix for a "Black Pyramid" is still not nice, but just get a long piece of something rigid that is: Disposable, Easily Cleanable or both. Step on the pedal and try to dislodge the solids in the tank bottom. From now on, use more water.
Now you have a guide that would have cost a lot of money at an RV dealer and you don't even need or want to replace the stool.
Matt