You can use the RV's built-in water pump to fill the FW tank if:
1. your RV came with a winterizing system that includes an intake hose on the tank side of the water pump with a tee valve that turns off the FW tank outlet and that opens up the feed to this hose and,
2. this winterizing hose can reach the container with your spare fresh water in it and,
3. you have an outside shower hose close enough to the FW fill opening such that the shower head flow can feed water into the FW opening as this water is pumped out of your spare water container by the RV's water pump.
Then - just turn on the RV's built-in water pump and let the outside shower add your spare fresh water into the fresh tank. This should work well, but I haven't tried it yet myself even though our RV is setup this way.