Tank vent line is blocked. 32 years ago they were the same size the industry uses now--5/8". Tank vents have two main purposes--
1. provide an escape for air in the tank displaced by incoming contents, and
2. provide a source of air to replace tank contents as they're pulled out
If no air can escape the tank, it will become pressurized as the toilet pumps more and more contents into it...this can have some unpleasant (albeit entertaining to uninvolved spectators) consequences...the least of which is a geyser when the deck
pumpout fitting cap is removed, the worst being a burst tank...and several things in between.
If there's no source of air to replace tank contents, the pumpout or discharge pump will pull a vacuum that prevents more than a gallon or so from being pumped out. Worst case can even be an imploded tank if the pumpout is strong enough.
If you pumped out and your tank is still full, it's a 100-1 odds thana blocked tank vent is the reason. Unless you want to risk any of the unpleasant consequences I mentioned, do NOT use the toilet again or pump out or dump--or let the pumpout service
near your boat--again untill you've done the following:
First open the deck fitting cap to relieve any pressure...I'd be sure to keep arms length UPwind of it when you do this. Backflush or scrape out the vent thru-hull (knock out any screen
in it...screens cause more problems than they solve or prevent). Then remove the vent line from the tank and clean out that end of the hose and the vent fitting on on the tank.
NOW you should be able to pump out the tank. To keep the vent line open, back flush it through the vent thru hull
by sticking a hose nozzle up against it (if your vent thru-hull fitting won't let you do this, replace it with one that does) every time you wash the boat and/or pump out. I wouldn't use the same hose that you use to fill your water tank(s).