The tank connectors should be able to be accessed at the front of the tank, where they would be disconnected before dropping the tank. There would be a short line that goes from the front of the tank to the fuel tank assembly. The fuel line connections on pwtrol models are quick connect fittings, but I don't know if the diesel are the same.
I'm wondering, since it's better when the tank's full, if the fuel filler cap is venting or allowing a vacuum to build up and sucking air in somewhere. Might be worth undoing the cap after each journey just to relieve any vacuum in the tank and see if that makes a difference at all.