Thanks guys. Seems I'm not the only one with at least some variation of this problem.
Gerald, it's a Magnum chassis but I only have one filler port on the passenger side, just aft of the front whee, above the belt linel. I got under there today. There is what looks like 1.5-2inch curved metal pipe about 8 inches long. This is the filler body and cap. It's connected to possibly a 2 inch rubber hose that goes down to the tank connection on the side, top of the tank, about in the middle. There is a small 1 inch hose, which I'm guessing is the vent, that runs up to the filler body, next to the filler pipe and connects to something that I can't see. The other end runs down, through the frame rail, to the tank connection which I can't see. Where would the check valve be? The filler tube has a slight dip, but it doesn't seem so severe as to cause this problem.