My son-in-law has a Salem trailer. The 3"pipe from toilet to tank looks like it goes to about 2" from bottom on tank. Therefore the toilet solids will not flow out in the tank. I think the pipe needs ...
Unfortunately trailers are all different and the pipe could be glued into a collar or only a slip fit into a bushing. You're probably going to have to pull the toilet and unscrew the flange to figure out which you have. 2 inches seems extreme, but if the solids are staying in the pipe then I would bet you have a slip fit that can be pulled up higher and trimmed off.