We filled our tank at Lion's Park in Moab - it's a little park on the river at the junction of highways 128 and 191. There's a threaded spigot in the lower parking lot that's they allow you to use to fill with. You'll want to loop the parking lot clockwise if you need to fill from the non-curb side of a trailer I just checked the Canyonlands website and it confirmed my memory that there's no potable water available in the Island in the Sky campground.

This site is about 30 minutes from Moab and 15 minutes from Canyonlands. You should be able to get water pretty easily.