My wife and I just purchased a pop-up trailer and are new to the camping world, so please excuse the ignorance of this question! Our trailer has a 16 gallon water tank underneath the floor and is filled by a gravity fill spout on the side of the trailer. However, when we attempt to fill up the tank only about 2-3 gallons go in before the water starts backing up out of the spout. We've been all over the owner's manual and I'm sure we're just missing something really obvious, but does anyone have a solution to this? Is there air in the water tank pushing the water back out and if so, how do we get it out? Any help will be appreciated!