Just another random thought but if your have wasps they will often get into vent pipes and cause issues. When we bought our used camper I was told that there wasn't a big problem with them in Colorado but in checking the slow drains, after cleaning the "P" traps, I pulled out a wad of nests. Stopped that by purchasing mesh drain screens that are supposed to go in sinks and gluing them into the top of each vent pipe before replacing caps on the roof. Now the only place they can nest is where I can see them and remove any would be invaders. Problem solved.