The better quality vent fittings have screens in them to keep critters out. You don't say what model boat you have, but you may have to install an access plate in the inner liner adjacent to the fuel vent/hose fitting.
What's more likely is that when boat is on the trailer, the vent fitting may not be at the highest point on the tank, so you're trapping air in the tank and it'll start spitting fuel out the vent before the tank is full.
On my Seafari the fuel fill is on the starboard side a couple feet fwd of transom, so if I"m refueling at a marina, the boat would be healed to stbd and down at the stern a bit since that's where I'm standing. The highest point on the tank is at the front on the port side, so that's where they put the tank vent (tank is original), with vent hose running from there to the overboard vent fitting in the hull back near the fill pipe, so you can tell when it's getting full. However if I refuel at a gas station with the boat on the trailer, the front of the tank is indeed lower than the rear and it evidently traps air in the tank, because even though it spits gas out the vent and tries to overflow at the gas station, once I get it home and jack up the front of the trailer, I can always add about another 5 gal before the tank is actually full. Sounds like you might be having the same problem! I just take a 5 gal. jug along when I get gas so I can top off at home or after I launch the boat.
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