It's a venting problem. We had the same thing when our rig was new, the vent pipe was extending too far into the black tank, but I'd have thought the previous owner would have addressed that. I've also heard of others having the issue after the black tank had been "overfilled", allowing TP to get stuck in the bottom of the vent pipe.
You can also check to make sure nothing is blocking or covering up the vent pipe where it exits the roof.
Jay & Peggy Monroe with Dolly
Can't take it with you - don't plan on leaving any behind
2016 Newmar London Aire 4553, Spartan chassis
Wrangler Unlimited toad