G'day Garry
Is the tank in the Edencraft a poly or a fabricated one ? . My custom alloy tank has the filler at the front so I have to use the jockey wheel to raise the front of the trailer at the servo if I want it really full . Have heard of tank baffles with insufficient flow thru causing slow fuelling while the fuel levels out but that is not the problem you describe . Any tank that is at least level should fill right up regardless of the vent line as the air is displaced out the filler neck IMO .
Chris