I had the toilet burping problem a while ago. It got steadily worse until one day, to my horror, I could see the peak of the "brown mountain of doom" in the drain pipe. The cause turned out to be a combination of things: I was dumping the tanks on a time schedule instead of waiting until the black tank was almost full, and I was not using any chemicals in the tank. After I had removed the brown mountain (let's not go there), started using chemicals and adjusted the dumping schedule, I've never had the problem again.
Bruce Dickson 2013 Thor Challenger 37GT, 5 Star Tune, Safe-T-Plus Steering Control with Air Trim, Roadmaster front and rear Sway Bars, SuperSteer rear Track Bar, Crossfires, 2012 Honda CRV on Blue Ox Aventa LX tow bar. Full timers since Jan 2012.