Update:

First I removed the the pump out deck fitting cap so I wouldn't build up pressure in the holding tank. Then, using low pressure with a water hose, I pressed the nozzle against the outside of the vent where it comes through the hull. (The vent hose coming from the tank goes above the through hull fitting about a foot and a half and then down to a 90° fitting through the hull.) I ran some water into the hose and after a few cycles of putting a bit of water into the vent hose and then giving it a rest, I figured that if there was a blockage, I probably got some water onto the outer end of it.

I then gave it an hour or more to soak (if indeed a blockage existed and I had gotten water to it). I then took the wet/dry vac to the outside of the vent hose through hull, prepared to possibly get an ugly reward for my efforts. The vacuum pulled out nothing but smell. (Maybe that is a good thing.)

I then had my wife hold some plastic wrap over the pump out fitting hole while I used the vacuum on the vent hose through hull. She reported a pretty good pull down on the plastic wrap from the vacuum. I believe that this indicates at least that the vent hose is at least moderately open.

We probably won't have another pump out for another 4-5 days so I won't know for sure if I have fixed the problem or just eliminated the vent hose as a suspect. (The fellow who was doing the pump out said that he thinks it was a vent problem.)

I appreciate all the suggestions and will update when we know more.

Nick