Some of the bigger truck stops, particularly Flying J. Some RV dealers, a few campgrounds might do it for free or a nominal charge, some rest stops, friends and family, and probably other places as well.
I would suggest carrying a bit of water, in case you want to stop and wash your hands, or have lunch or use the toilet. I usually carried 1 mark on the guage worth of water in my tank, about 10 gallons I think.
Also carry a water filter. We went into a campground in PA once that had water so bad, you could not drink the coffee made with it. We use a carbon type that goes in the water line coming in and replace it once a year for about $23.00.
yes, I like a filter which handles down to bacterial size just to get into the RV. I would want a 'purifier' (down to virus size or better) for drinking water but those are pricey, so I carry a few gallons of purified water for human consumption.