It definetly sounds to me like it's blocked tank venting. The only source of water
filling the tank during pump out would be the result of a vacuum drawing water
thru either the head
intake or an overboard
discharge line. Close both of these thru hulls before pump out. If the head
drains during the pumpout then blocked venting is the most likely culprit. This could happen anywhere in the line from the tank to the vent itself. I'd recommend disconnecting the vent line at the vent and then, using your dinghy
air pump, see if you can blow air back thru the line to the tank with the pumpout fitting open and then thru the vent itself. The airflow should unrestricted.