Hello All! It has been a while... I hope you are doing well!
When it is just my wife and I camping we have found that we can go at least 10 days and even two weeks without dumping the black tank. We rarely have sewer connections at the places we go and I hate using my little 20 gal tote to dump black water. If we can make it until departure, I much prefer to dump on the way out.
I noticed very early on that once the black tank level hit 80-85% (on my SeeLevel gauges) that the water in the toilet would bubble up when flushed. To me this meant the black tank was not venting properly. This was, however, a minor inconvenience as it never burped hard enough to splash anything out of the toilet (thank goodness!)
When the weather is warm however, cool in the morning and warmer during the day, I started to notice the black tank foul odor in the basement and (less so) in the bathroom. Again, this is a result of the tank not venting properly to the roof when quite full. With warming temperatures those odors had nowhere to go but back into the unit.
I suspect that when they installed the vent pipe into the black tank that it protrudes into the tank 1 - 1 1/2" reducing the tank's effective capacity and causing venting problems when nearly full.
The only thing I could think of to solve this was to crawl into where the piping is "accessible" (for a contortionist!) behind the back wall in the basement and add a 3"x1-1/2" Tee to the down pipe from the toilet and connect this to another 1-1/2"x1-1/2" Tee added on the vent line. Fortunately, the vent pipe from the black tank is only about a foot away from the 3" down pipe so my horizontal connecting piece did not have far to go. I was careful to place the Tee facing up on the 3" pipe to prevent any liquid from getting into it and I then went up further with an elbow to be sure the liquids go only down to the tank. To install this 3" Tee I removed the toilet and unscrewed the flange from the floor. I could then cut it twice to make room for the Tee and my son lifted it up while I glued the Tee into place. The vent line was easier to install the smaller Tee.
This has worked out great! When the tank is quite full and the vent line is blocked by the liquid level, the vapors can vent through the 3" down pipe through my new cross pipe arrangement back into the vent pipe and out to the roof! I can now use 100% of the tank capacity with no burping in the toilet and reliable venting all the time. I have not had any odors since. Happy Camper!!
Sorry I did not take any pictures, it is so hard to get in there. I hope this explanation is clear.
Best Regards, Mike