Our toilet started burping when you flushed it.  Not very pleasant. 

That's a good indicator that your vent stack is plugged up.

Rather than removing  the 10mm of adhesive securing the vent stack cap, and then messing around with a snake (ewwww), I made a tool.

It fits over the entire vent stack cap and against the adhesive, which pressurized it when I turned on the water.

Problem solved, 5 min, no mess.

Make sure the drain is open when you do this, as you are pressurizing the only inlet, or what ends up in the black tank would probably end up in the RV.