Originally Posted by denloe1
A defect my Grand Banks had that is apparently common was that the copper dip tube into the holding tank which routes to either the pump out or macerator, normally was about 18" long and went nearly to tank bottom, in my case was corroded away to a 4" stub. About 3 gallons is all we could get out. Replaced all tank fittings with new PVC ones from a Seattle marine sanitation company after doing the sludge removal regimen and lots of flushing out. Now we have a nicely working 35 gal system.
The symptoms of a clogged vent or a broken dip tube would be very different. Anybody who understands a marine holding tank system (and it's very simple) would have figured out the cause of the OP's problem in just a few minutes.