I have a Coachmen as well, and wanted to be able to fill my tank from some 5 gallon jugs. I thought I could just hook up a short hose and use a funnel to fill it through the freshwater fill, but there's a backflow prevention valve or something there that prevents it from filling unless the water is under pressure.
So I bought an RV freshwater pump, attached hose fittings and wired it to my batteries, and used that to pump water from the bottles into the freshwater tank. I keep the pump around as a spare in case the one in the RV fails.
If the hose you're filling from has good pressure, you might be able to use something like a
water bandit to slip over the water source hose.