Problem: pressure builds in the black tank and when you flush a big air bubble splashes water from the bowl all over.
I had this issue and after searching for a while trying to figure out the problem, I finally did.
I noticed this when I would be dumping or cleaning the tanks in my driveway. I would back in, connect the dump hose and go though all the procedures. While rinsing or winterizing, as I had the black tank valve closed and ran water down the bathroom sink and then flushed the toilet a big air bubble would splash water from the bowl out all over the floor. This never happened when out camping, only while backed into the driveway.
First I thought there was a clog in the vent pipe and ran a snake all the way down with no improvement. The black tank is already sparkling clean (no poop pyramid).
What finally turned out to be the problem was the long run of drain pipe running from the middle of the black tank (under the shower) behind the toilet, into the lower part of the vanity and up an interior wall behind the sink and continued up through the roof as a vent. From tank to vent was about 4 or 5 feet of mostly level drain pipe. Oh sure it had a bit of slope when I was park level but when backed into the driveway there would be enough slope in the wrong direction so that when I put water down the sink it would fill the pipe blocking the vent. As I put more down it would force it way into the tank creating more pressure until water would eventually back up into the sink. At this point if I flushed the toilet a bunch of air pressure would splash water out the bowl as a big toilet burp.
If I opened the drain all the pressure would be released and the sink drain and no burp.
After read a lot of complaints about this with many different possible causes I thought I would give my solution.
I ended up rerouting the drain pipe to add a lot more slope. This wasn't an easy fix but works and gives me a bit of peace of mind that next time I de-winterize I won't be making big messes. I really didn't want to do nothing and hope I was always parked level enough to not have this happen while out camping. I would never get my wife in the RV again.
I know this when long but I thought it might help someone struggling with the same problem and instead of it being a poop pyramid to consider it may be a bad design in the plumbing. I now have a least 10 degrees of slope or more from the sink to tank. This is plenty to overcome my driveway slope and I hope I never try to camp that far out of level.