For NV DMV the requirement is a receipt with the RV park's address for one month's rent and an acceptable mailing address. Commercial mail forwarders will not work.
Which RV park in Pahrump has a 28 day limit? Sounds weird.
One guy on this site used a RV park off West Basin, from NV-160 go past Blagg and there is blue sign on the right to turn left to that RV park. He was building out a box truck, so they won't care, unless your rig is a mobile meth lab, than again maybe, maybe not.
The RV park is about a couple miles from Home Depot on NV-160 which was handy for his build. He got his Nevada Real ID DL and was good to go, which he did. Last heard he was getting a campground job in a forest somewhere and never checked back in.
Nothing has changed in the five years that I have been here in Nye County. Except Pahrump has gotten a bit less polite.
There are a number of prior threads with a multitude of posts about NV DMV, mailing addresses, and Pahrump.
Use the forum's search function.
Be wary of recommendations to use an unoccupied house as the street address for your mailing address. Unless you like to do that sort of thing.