Not having enough water in the back tank is the number one reason for black tank problems.
Dry campers have the absolute worst time with the black tank because they have a tendency not to use enough water when they flush (trying to maximize time between dumps). This is false economy because the solids will pile up under the inlet for the tank and become "the mountain of doom" blocking the dump valve.
They also use a dump station that has a line (typically), and being the nice campers that they are, feel bad about using the Turboflush (if installed - and it should be!); so they don't. This makes filling the black tank to the "full light" even more critical when you head to the dump station.
Lou and Laura with Bella - German Short Hair Pointer
2008 GMC Sierra 2500HD Crewcab SB Allison Duramax
2010 Flagstaff 8526RLWS - Superglide 3300
HAM CALLSIGN - KC3FFW