Your tank vent is blocked.

Air in the tank displaced by incoming waste and even air pumped through the system has to have an escape route...which is the vent. If the vent is blocked, the tank becomes pressurized...which will cause all kinds of problems up to and incuding a burst tank...at the very least, leaks at fittings...backups--even eruptions--in the toilet... So clear the tank vent and you'll cure the leaks.

The vent is also the source of air INTO the tank to replace contents as they're pulled out.. a blocked vent will cause the pumpout or onboard pump to pull a vacuum that prevents more than a few ounces from being pulled out. A particularly strong pumpout pulling against a blocked vent can even implode a tank.

The two most common locations for a vent blockage are the vent thru-hull and the connection to the tank--both that end of the vent line and the tank fitting. Clean 'em out--scrape 'em out with the screwdriver blade or whatever works. And then KEEP the vent clear if you want to prevent all kinds of nasty problems.