This may have already been tried, but I would be suspect like you say of either a clogged inlet in the tank, bad fuel hose / line, or what I am wondering because this really sounds like the problem, do these trucks have inlet screens in them? I know a lot of the heavy equipment I have worked on have inlet screens on them, before the fine partical fuel filter, and in time they can gum up and when they do, the engine performs exactly like you're saying.
My bet right now is on a clogged pickup or something like that in the tank. I can't blow anything from the tank forward using air pressure like I should. I'm siphoning the tank as we speak and through the clear hose I do see some trash coming out of the tank.
