Symptoms of clogged vents are lingering bad odor, leakage around tank fittings, hissing noises, bulging tank walls, liquid flowing back into the bowl, and difficulty emptying the head.
This is a simple thing to check and fix. There are two options to handle this matter:
1. Remove the vent line from the vent fitting on the hull. Use your dock hose with a nozzle capable of producing a strong stream and flush the line into the tank. If the water is flowing freely, the partial obstruction will be cleared.
2. Replace the vent line. Remove the vent line at the hull fitting, and the vent fitting at the top of the tank and install a new line. This is an easy procedure that only requires new hose and clamps.