How much water do you think you could safely carry on your roof?
It would depend on your camper..We all have seen some with half the town standing on a TC roof and the cargo boxes/canoe's/firewood etc etc.
Another option when water is limited is bringing outside water from creeks inside to heat up shower and clean up.I use a large canning pot to heat up creek water to use as a shower.
There is also the 6 gallon water containers for extra water that can easily fit behind your truck seat/just inside the TC door or on a hitch haul.I couldn't count the number of times I have filled an RV from a 6 gallon water container out in the boonies.