I may not be understanding your problem, but if I don't have access to a hose to fill through the convenience center I fill through the tank vent which is in a black plastic cover on the street side near the rear of the trailer about waist high.  Remove cover by pulling up and out, then remove the screen and use a funnel to allow you to pour from a  container into the tank. It will take a while if you try to completely fill a 40 gallon tank and is difficult to hold even  a 5 gallon jug up that long so I set up a table and block up on it high enough to  hold the container up.  Some campgrounds we use have faucets spaced out through the cg so I carry 200 feet of zero gravity hose and a "water thief" in case the faucet is not threaded.