As far as the toilet, most likely it is just toilet paper that has jammed into the slot and needs cleaning out. Common problem on the old Thetford. You can remove the toilet, flip it over and remove the valve. Once you get the valve, you can clean the slot and lube the rubber gasket. Very likely it will work nearly fine after that.
While you have it off, you can take a good look at the seal, and replace that if necessary. It also may be cracked where the flange meets the tank. Most tanks are made from ABS, which seems to age and crack after a while. If that is the case, it is unlikely that any repair will last. It is difficult to get anything to stick to the tank, and yet stand up to the flex. Go to TankDepot.com and start shopping for a tank. If they don't have a premade tank that will work, they also will sell a straight rectangular tank, and then will mount fittings where ever you specify. Been down this road, and was able to replace my broken 27 gallon grey tank with a 40 gallon.
'83 Revcon Prince 31' FWD FMCA F298817
502 w/Howell/GM ECM/Edelbrock MPFI,Thorley's & Magnaflows, 4L85E 4 spd, KoniFSD