I can't imagine why a P trap would be in your gasoline fill pipe. In general, you want a continuous slope from the cap to the tank with no kinks, or obstructions. The vent line (a smaller line, maybe an inch or so outside diameter) should also slope from the connection just below the fill cap to the tank. This is VERY important. If it does not slope it will get gas in it in a pocket and that will prevent the air in the tank from venting out. The fill will then be erratic or exceptionally poor. On my '83, 310, when I had it, the vent line had a dip in it and you could not get the thing to fill worth a damn. I re routed the vent so that any gas in the vent tube would run back into the tank, and the problem went away. I had a Jeep Grand Cherokee with the same issue and the same solution. That is one reason I asked if your gage read "full" when you were finished. The Jeep, especially, would not fill all the way with the dip in the tank vent line.
Tell more about this P trap you found in the fill line. Sounds like some PO has been at it again...LOL.