My fresh water tank is near my ladder, and I can use a bike tie down strap to hold the tank in place, and a syphion to get the water from portable tank into the fresh water filler.
My plan for full timing includes a 15 - 35 gallon fresh water tank in the back of a SUV towed car, and a 12 volt pump!
Tank-Depot.com sells a wide variety of tanks. . .
My vote would be to buy another 12 volt pump! Then such the water out and into the fresh tank.
You can also buy a 3 way valve, install that in the intake line for your pump. If you have the ability to open a valve, and then direct shore water into the fresh water tank, this might be possible, then install a 3 way valve, and short tubing to go into the portable tanks.
Good luck,
Fred.