..say, more than a week or two, it's best to pump out AND rinse out the tank. If the boat does sit with waste in the tank, it's even more important to rinse out the tank after pumpout. However, from your comments, it appears that storing the boat with a clean empty tank may be a challenge...so we prob'ly oughta address how to deal with a half full tank sitting for a while...and venting is the key to being able to do that.

How big is the tank? How big is the boat? Sail? Power? Where in the boat is the tank located? How long is vent line? How straight--or not--is it?