Keith wrote:
Easiest way to figure that out is to remove the cover to the pump-out
and see if that fixes your problem.

It may work, it may not...'cuz the vent fitting on the tank is above the
surface of the contents, while the pumpout line comes from the bottom of
the tank. It would take considerable pressure buildup to create enough
pressure from the bottom of a tankful of waste to push the air ABOVE the
contents being displaced by incoming waste out a line that may be
several feet long, and vertical to boot.

Don, if you decided to try it, open the deck pumpout fitting VERRRRY
carefully...'cuz if your tank is seriously pressurized, it could produce
a very unpleasant geyser. Any pressure will produce at least some
outgassing (hissing) when you open the cap, so you won't have to try
pumping the toilet to know whether the tank vent line is blocked.

--
Peggie
----------
Peggie Hall
Specializing in marine sanitation since 1987
Author "Get Rid of Boat Odors - A Guide To Marine Sanitation Systems and
Other Sources of Aggravation and Odor"
http://shop.sailboatowners.com/books...ku=90&cat=1304