Check tank vent for a blockage
The vent on all tanks--water, fuel, waste holding--has two essential functions: it allows air IN to replace contents as they are drawn out, and it allows air displaced by incoming water, fuel or waste to get out. Therefore, it's absolutely essential that all tank vents be kept open, because unless air can be pulled in through the vent to replace the waste (or anything else in ANY container) being pulled out, a vacuum is created in which nothing more can be removed--but in which the suction CAN crack the tank. And a pressurized tank can have equally disastrous consequences--anything from a geyser when the deck fill or deck pumpout cap is removed to a backup in the toilet, to blown out fittings, to a cracked tank. Since most blockages do occur in the through-hull, that’s the first place to look. Blast with water; if that doesn’t remove the blockage, use the tip of a screwdriver to scrape it out. The next most common place is the connection on the tank. Remove the hose and scrape it out if necessary.If there’s no blockage in the through-hull, check the vent hose for a kink or sag in which water or waste has pooled. Remove the vent hose and clear any clog by blasting water through it (or, it may be easier to replace it). Shorten it to remove any sag.