There are several ways to clean the vent line out; compressed air works well!
But, when you have the tank PUMPED OUT at the marina: have them over fill the tank several times to clear out the vent line.?
Another good trick I learned when having issues with the GROCO head: have the tank emptied just before working on it! As they are pumping out the head, add some fresh water? to the GROCO BOWL ( I use the shower head and add fresh water ). It is alot easier to work on the head plumbing after it is pumped out - the pump will suck the water right out of the hoses.
Daryl
s/y Intuition - PSC 31 #54
Daryl Clark