Many RV parks have a pool with a spa. I would think RV parks would frown on you filling up the tub and then dumping it when you leave. Plus, you won't have any privacy - all the kiddies would be visiting you and the older folks would have their binoculars riveted on you! There would also be a liability issue for a child falling in when you weren't there.
We do know of a couple who filled their portable in Quartzsite, AZ while boondocking in the desert. It took many trips into town to fill their containers with water to haul back to their tub and then they rigged a heater powered by solar.
Take advantage of some of the hot springs as you travel...a lot easier.