Why can't one use the water pump built into their RV?
1. Install a tee valve on it's input with a 25 ft. (or shorter, as needed) piece of the clear, small diameter, potable water flexible tubing coiled up and stored right next to where the water pump is. Normally the tee valve would of course would be turned so as to direct the pump input to the RV's freshwater tank.
2. To replenish the RV fresh water tank, turn the tee valve handle to the other position so the water pump would draw water through this piece of clear tubing that you had running into whatever tank your had your extra water stored in.
3. Now temporarily screw onto a sink faucet in the RV a piece of potable water hose (white color?) that could reach outside into the RV's fresh water fill opening.
4. Turn on the water pump and fill the RV's fresh water until it's full or until the extra water tank runs out.
Wala ... make use of the existing high quality water pump built into the RV that is already prewired with an ON/OFF switch and that is powered by the RV's batteries!