Re: ATF: Road Warrior - Toilet Belching
You did not mention, is this from day one, after first use of trailer or after several trips?
We have a 3010, same floor plan as the 305 and that is not normal. We seldom even come close to filling the tank enough to get the "tank almost full" burp.
I am with the above suggestion that you have a restriction somewhere between the bowl and the tank, or right at the tank, or a vent issue.
That is a 3" pipe going down, angling 45 degrees and continuing to the tank, so you can't see straight down.
But, because it is a 3" pipe a 5 gallon bucket of water should dump very quickly. Have you done that yet?
I would suggest for starters:
Flush several times to get as much clean water in the pipe.
Fill the toilet to the top and dump once or twice.
Fill the bowl right up to the top and dump several times
No different? Connect to the sewer discharge and try again with the black tank drain open to the the sewer connection.
If that helps or solves the problem that could narrow down that you have a clogged vent issue as you are not only trying to vent to the roof but are also venting in the campground's sewer system.
If you are comfortable enough that you will not get a "facefull", fill the bowl to the rim, have a 5 gallon bucket filled and dump both as fast as possible.