I believe the Maverik gas station on the south end of town has a water spigot for RVs. You may call them, though.
There are also some RV parks/resorts that will usually let you fill, for a fee. Still, if you want to travel dry, it is a small price to pay to fill when you get to Moab.
You say you will be boondocking. If you are up for a treat, we stayed at the Porcupine Rim Campground, 38.575542, -109.345886. We had a smaller trailer, but you can camp in the campground with most rigs. You can hike to the point, and have great views.
2017.5 Lance 1995
2017 F150 EcoBoost, Max Tow
Most camping off-road