[Chris Burkard]

He mentioned that he tried pumping overboard and at a pumpout station and there is still waste in the tank. That tells me he had the valves in the wrong positions.

I have seen a Y-Valve in the wrong position when trying to overboard discharge invert the duck bill valves.

Scenario - Thu-hull open, Y-Valve in the to deck fitting position, turn on the discharge pump. The pump pressurizes the lines, but the Y-valve is in the wrong possition with the cap on and waste can't go anywhere. When you shut off the pump, the pressure in the lines, forces the waste back towards the tank and inverts the duck bill valves.

The only scenario that I can see where he could not pump out through the deck or overboard is this.

Thru-hull shut, Y-Valve in the (from tank to overboard discharge). This would keep you from pumping overboard and from pumping through the deck fitting at a pumpout station.

A clogged vent line could keep you from being able to pump the tank out, but if the tank is full enough to back waste up in the toilet, some waste should have come out when trying to pump out at least until the vacuum in the tank overcame the pumps ability to suck the waste out.