When we first got our boat the vent line was apparently plugged at the connection into the holding tank. Turns out there was a small diameter stainless elbow
connector from the vent line hose into the tank. The elbow was plugged. Result was nearly a catastrophic explosion of the full tank. As the head contents were pumped into the tank, the residual air became pressurized and the tank actually swelled up.
Fortunately we caught it just in time, removed the offending elbow and replaced with a 1 inch diameter plastic fitting and larger diameter vent line. You might want to check the tank end of the line because that is where sh1t happens!!