Mark
This is for sure a vent line blockage. First i would look at the vent line where it exits the hull and make sure the it is not blocked. These can get packed with mud from wasps etc. pop the creen out if the vent if it still has on, then poke a wire into the vent to clear it from the outside, note that the vent fitting is a 90 degree smooth bend into the hull. On the inside it will then have a rubber host to the holding tank. Get this vent unplugged.
Next solution is to go to a pumpout station and put the pump onto the pumpout fitting. Then when evacuating the tank, you should hear lots of air also being sucked into this vent fitting. At this point take a water hose and spray water at this fitting allowing it to suck water into the vent flushing it as well.
At this point your vent line system will be clear and the problem will be solved.
Let us know how it goes and if this fixes the problem.
Hayden
