We did not see any hydrants for filling RV tanks at the campground earlier this month. The only water tap was at the restroom utility sinks. I would recommend filling up in Moab. Water supplies at the campground are limited.

There is a hydrant for potable water just outside the entrance to the camp ground; right alongside the small parking area for the day-use, picnic area. There is one small problem with the water hydrant, if I remember correctly. That problem is that I could not run a water hose directly from the hydrant to the fill port of my RV. Instead, I had to fill my 5 gallon water carrier from the hydrant, and then pour the water from my water carrier into the fill port of my RV. It was a hassle, but it worked. Perhaps the hydrant is more adaptable now. I don't know.

We filled ours and did a dump at a full hook up site in town the night before our stay there, I don’t remember any water at the entrance or anything. There is a place in tow that allows fill ups for non guests for a fee. Can’t remember the name but it was on the main drag opposite end of town.

There is not anywhere to fill RV water tanks within the park itself. There is a dishwashing sink in the campground, but it is not for filling tanks (although you could try, we found the camp hosts very present though). We camped at a full hookup campground the night before and made sure our tank was full before we left.

No, not nearby. Nor is there a dump. But on the east side of Moab is Mavericks Gas where you can dump and fill up with water. There are other places, but they will charge you. https://www.discovermoab.com/rv-dump-stations/

If I remember correctly, we filled our tank before entering the park in the town of Moab.

Get a "Water Bandit" to increase your ability to fill from different sources. There is no official/sanctioned fresh water fill for RVs within Arches NP itself, but you just have to decide how creative you wish to be. A low-tech 5-gal jerry can with appropriate nozzles will do wonders if you are willing to do the legwork. As mentioned, if you are going out to Moab to dump anyhow, the Maverik at the south end of town is free dump/water... but depending on time of day, the line can be long. Good luck!