Toilet Bubbles when Flushed

[RTinAZ]

Someone said to check vent pipe, thank you, but check what? Was just pumped, working fine. Gets 1/2 full and big time bubbling.
What am I checking for?
Using 5* Tank Tech for RV toilet.
In rural WI, no RV plumber for 75 or more miles.
Using appropriate Scott RV TP.
RTinAZ

 

[J&JD]

There is a black tank vent that extends up through the roof. Many times animals/bees/birds block the vent so that the toilet now becomes the vent and bubbles up. Get on the roof and check out the vent pipe!

 

[Gronk]

My question is HOW do you know your black tank is 1/2 full? Are you going by the tank gauge in the RV? They are notoriously inaccurate.

 

[FL-JOE]

The OP has two identical treads going on the same subject. Confusing.......