Vent pipe has slipped down into tank and is getting blocked with very little 'stuff' in tank.
Blocked vent...toilet burps
If vent line has not slipped down then it is blocked due to birds nesting etc
Either way you need to go up on roof.remove black tank vent cap and check:
Is top of vent line even/close to same level as roof
NO...has slipped down .need to pull it back up and secure it (couple screws thru pipe into wood
YES.then need to clean blockage---run garden hose down vent blasting away with strong water pressure (black dump valve OPEN)