First off...does your unit have a winterizing bypass with a length of hose attached to it...should be right at your pump...or you may have an exterior antifreeze inlet by your city water hook up. If so, you don't need to add antifreeze to your fresh water tank and can siphon directly out of the antifreeze jug. But seeing as you already added to your tank, you could just carry on with that. The problem you are having is that a gallon of antifreeze in that large freshwater tank is not enough and you are sucking up air. By sucking directly out of the jug rather than your freshwater tank you will need quite a bit less antifreeze.
Did you open up your low point drains? There are a lot of great youtube videos showing you how to winterize.
2011 GMC Sierra 2500HD
2014 Rockwood 8289WS, Diamond Pkg.