Add your own gravity fill inlet. It can be done by adding an inlet fitting to the tank with about a 1-1/2" dia. hose going to an exterior fill port with cap and door. Parts are available from RV tank suppliers and RV parts suppliers.
Also, if there is an unthreaded campground water spigot you can hook a "water thief" adaptor to your hose and push it on to the spigot.
Gary