RV's with two vent hoses out the bottom, usually have no gravity fill port and are pressure fill only.
A gravity fill is a cap that can open and water poured in, and have a vent next to the cap. We are not discussing this type.
The pressure fill type have a valve that you turn to direct water from the city water connection and send it to the tank. Air is expelled out the vent pipes that are all too often just straight out the bottom. Two pipes are used for redundant safety purposes. Plugging these vents, or using a "small hole" is not something you should do. The tank (which is not rated for pressure) can expand and do structural damage. Those vent hoses need to be routed correctly as I previously posted.
My tank now has both a gravity fill and pressure fill for convenience and safety. They are in a locking hatch with venting drilled thru the door.
