Fresh water tank fill problem

The problem like that on my FW tank is that the fill connection (not gravity feed) and vent are at nearly the same place on the side of the tank. I raised the vent hole where it exits on the side of the trailer to a level above the FW tank by removing the outside shower and extending the vent exit hole to that space. Now it won't siphon out. Previously, the siphon action would happen running down the road. I would fill the tank at home and only have half a tank left when arriving at a campground an hour away.