It might take a bit of fabricating, but you could cut your water line after the water pump and put in a Y-valve. One line would go to the original flow, the other could go to the fresh water tank.
You'd also need a way to tap into your fresh water tank inlet hose, and then make sure it also had a check valve so when running the pump into the fresh water tank, it would go into the tank and not out the fill port on the outside of the camper.
Then, when you added the water to your tank, you can flip the valve back to it's original flow.
A little common sense and some patients, no reason why you couldn't rig up something like this.