On my last rig, I did what 5picker suggests. I would suck water in using the camper's onboard pump using the anti-freeze inlet. I would then use the outdoor shower and connect it to the freshwater fill (in my case it was still gravity fed). The new camper isn't setup the same way.
We've been rallying with 50 families. I've lent my equipment out a bunch. This pump has pumped a couple hundred gallons of water this week from bladders to campers:
https://www.harborfreight.com/12-vol...ump-63324.html
I don't have any idea if it's drinking water safe, but figure it's going quickly enough through the pump to not matter. All of our drinking water is either bought or runs first through my ClearSource water filter and then through our Berkey water filter.