You most likely have what's called at TP teepee. When using an RV toilet, it's really important to flush enough water with any solids that they don't build up and plug the toilet. Basically, your toilet paper and human solids have formed a plug that has backed up into the pipe. But all is not lost.
You need a fairly rigid piece of hose to ram down into the blockage. Keep running it into the blockage until you hear the whoosh when every thing slides down into the tank.
Then, fill the bowl with water and flush it 3-4 times. Make sure you completely fill the bowl each time. This will rinse most of the solids into the tank.
Then, from now on, explain to anyone who uses it. If they are flushing any paper or other solids, that they must completely fill the bowl with water before flushing. This will prevent the dreaded tp teepee.