Another possibility in SLC is the Pony Express RV Resort. A little out of town and lots of residents there. Haven't stayed in either this or the KOA, so can't tell you if they're any good. When coming in April, avoid the first weekend in SLC, that's the LDS Church general conference and everything in town is full. if you want to see downtown, KOA also has the Trax (light rail) right outside the gate. It goes all over town and connects with the bus system. You can get all day passes for a pretty reasonable rate.
In Moab, we liked the
OK RV Park and Stables. It was a little south of town, but off the main drag and quiet.
Sounds like an ambitious trip, have fun and enjoy
