I'm a Caveman.
30 extra gallons of water could be carried in 6- 5 gallon portable jugs setting on the floor of the RV. (If you could talk her down a few gallons 5-5 gallon jugs would be easier to find room for.)
Then when you get to where you are camping just set the jugs outside on the ground until you need to add some water. A 12 volt electric pump would make putting them into the holding tank easier.
Coleman makes a nice square 5 gallon jug. But, their are cheaper ones that work well too. Just get a good one that won't leak.