When is the tank bubbling???
You say "while docked"...does that mean you're only using the tank at the dock? Does it happen after each flush? Only immediately after returning to the dock? Is the tank overboard discharge thru-hull open? Till I know more, I can't imagine what would cause a tank to "boil" for up to 10 minutes! Sudden thought: is it possible that the P.O. installed some kind of aerator in the tank...and that the pump starts when you get back to the dock and reconnect to shore power? T 'Cuz the source of the air that's creating the bubbles can't be above the surface, it has be low in the tank...well below the surface. A correctly installed aerator would explain it.The vent line is the smaller hose coming off the top of the tank or on a side at the top. The other end exits the hull somewhere...you'll have to trace the line to find out where. Or depending upon the size and age of your boat, it may end inside a rail stanchion. However, if the vent is blocked, flushing the toilet wouldn't cause it to "boil," it would pressurize the tank. Flushing the toilet wouldn't cause it either, unless you're flushing some product that's effervescent each time you come back to the dock. However, I don't think you're pumping the toilet enough times to move bowl contents all the way to the tank, which leaves waste sitting in the line...which will cause other problems. And you've got the wet and dry sequence backwards. I suggest you check out the link below.