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.