gas cap vent

[NOTHING ELSE MATTERS]

That doesn't sound right. Maybe they used the second pick up(some times tanks have a second pick up or return for diesel) instead of the original vent pick up. Check to see if you have a fitting or a pluged hole next to the tank fill (on the tank), if you do, that should of been the vent.

[macojoe]

It has none, maybe the put the tank in backward when they built it??

[NOTHING ELSE MATTERS]

No, tank fill is forward, so is correct. If tank fill is located in the middle, then yes, is installed backwards.

[macojoe]

well then its correct, I just need to fix this issue some how

[nymack66]

I like the idea of a Y connected to the filler hose, If not drilling is plan B, Just a FYI my filler and vent is towards the transom on my Boat I flip the tank around to make it work, I have no vent issues, my plan was to drill a new vent if I needed to.
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[phatdaddy]

since it happens only at rest on the trailer, leave the cap "loose" and see if the problem goes away before drilling.

[chumbucket]

He has already tested by leaving the cap loose and it corrects the problem. Obviously you can't leave the cap loose all the time because moisture will eventually accumulate in the tank.
I did find a vented filler neck that should alleviate the problem.