[flybouy]

In May's defense, if the trailer was purchased used and they never used it for "solid waste" then it may have been the previous owner's doing.

That said you have been given good advice and irregards to who made the mess it's yours to deal with. The other option if you are connected to full hook ups is to get a toilet wand which is a length of plastic pipe that screws onto a garden hose and has a hole in the other end to direct the water. You could pout that down the commode (USING A DIFFERENT WATER HOSE, DO NOT USE YOUR HOSE FOR SUPPLYING DRINKING WATER) open the dump valve and let it run.Periodically rotate the wand to direct the spray in another area and monitor it.

If you don't have one now get a clear sewer hose connection and connect it inbetween the camper and the sewer hose. They are about 5" long and around $8-$10. That will allow you to monitor how well it's draining and if the solids are breaking up and moving as well.