Since you are long time campers with limits to how much water you carry, you will likely do just fine with the onboard water tank.
I always carry an extra seven gallon jug of water with me, but instead of refilling the fresh tank from it, I usually just use water out of it. I keep a easy carry clear one gallon water container in the camper, and just step out to the tailgate of the truck and fill it from the seven gallon jug. I use that for things like making coffee, refilling the dogs water bowl, etc.
Here is a site that shows many dump sites:
http://www.rvdumpstations.info/