What do you mean by "from gallons of water"? A 5-gallon jug like you'd mount on a water cooler? A collapsing water jug?
The RV should have a cap for the fresh tank that you can unscrew. Just use a funnel
The CDC recommends having 1 gallon of water per person per day in emergency situations. You'll be in the desert, you'll be cooking and doing other things that aren't done in emergency situations, so budget 2 gallons minimum per person per day. You will almost certainly run out of water if you're there for the duration.
Navy showers will help conserve some water.