Agreed. You may have a Y valve on the discharge. You can also follow the hoses from the tank. The larger (1 1/2") hoses at the top of the tank are the 'Fill' hoses from the head. The larger (1 1/2") hoses at the bottom of the tank are for discharge (overboard discharge or pump-out). The smaller hose(s) at the top of the tank would be the vents.
If there is a straight shot from the bottom of the tank to the pump-out deckplate, then it could be either:
1) discharge hose is blocked.
2) Vent is blocked.
I'd start with the vent