Just got off the phone with Merlin on a different issue so I asked him about yours.

He said that the most common problem associated with being able to fuel up is actually the Cut Valve Assembly (commonly referred to as the Charcoal Canister) located on the fuel tank. List price is $126. He's basing that on his experience with Nissans since he's not had any calls for Isuzus with that problem.

He also indicated that the Roll Over Valve is located on the tank (not in the fill tube like I assumed) so it should be a replaceable item (although he's never heard of one going bad).