It could be the toilet flange-shaft that you look down that goes into the black waste tank.
Remove the camper from the truck, there is an access door just under the sink and you should be able to get a look at the front of the tank. What is happening.
Take the toe kick off of the step up into the camper. You should be able to get a look at what is happening.
I had this problem and it was the toilet flange neck. It was loose and I replaced it. Pull the toilet flange out with the drain pipe-neck. Not that difficult, To reinstall it use plenty of silicone to seal the shaft-neck into place. Be sure to position and install the toilet bolts before you finalize things. You may have done this, but its important to pull the neck out, its difficult to get a good seal without pulling it out.
If its something else the above should help you to know what is happening. You may want to give Bob at Outfitter a call, they have been helpful to us.
We like our camper as well but as with all equipment it does require maintenance.
Good Luck.
Rick
