Another thing to check is the air vent to the holding tank. It can get clogged in at least two ways: from the inside if you flush too much into the tank (a wad of toilet paper makes a pretty good stopper), and from the outside if a mud dauber wasp or some other insect builds a nest in the vent opening. If the vent is clogged, you're creating a vacuum in your tank when you pump it out. It might not collapse, but don't chance it.
To clear the clog, get a toilet plunger. Remove the handle and place the rubber plunger over the vent, on the outside of the hull
. Press a water
hose on the hole where the handle was, and blast the vent with water. If that doesn't work after a few tries, you'll have to figure out how to remove the nut attaching the vent on the inside of the hull
, so you can remove it and clean it and the tubing.