I don't think it's a problem with the toilet, but a blocked tank vent COULD prevent the pumpout from removing all the waste.
However, even if the problem IS a blocked vent, I suspect the real problem is incorrect USE of the toilet and lack of maintenance
by the owner(s).
Portapotties have a flush water
reservoir that adds at least enough to rinse the bowl with each flush...do you put any water
in it? If you don't, that would certainly explain why solids and TP aren't dissolving...urine alone doesn't provide enough liquid in the tank for 'em to dissolve IN! You HAVE to start to putting some water in the tank. If you don't want to fill the reservoir, add it directly to the tank. Not a lot...a quart should be enough.
As for maintenance
, the tank vent should be checked regularly and the vent thru-hull cleaned out as needed. And you also need to start rinsing out the tank at least occasionally, just as a matter of hygiene! Use a shop vac to remove the rinse water.
And fwiw, a 2.5 gallon tank holds so little that it actually makes more sense to turn it back into a portable that you can carry off the boat and pour into a toilet, which would also enable rinsing it out after each dump. Otoh, a 5-6 gallon marine
"MSD" portapotty will fit in the same footprint, but holds 50-60 flushes, compared to the 20 your 2.5 gallon tank holds and is actually designed to be permanently installed and fitted for pumpout. A google
search for "Thetford 365 MSD" Porta Potti 365 MSD
turned up a whole BUNCH of retailers who have it for as little as $117.