Escapees is the only organization that does what they do. But you do not need to have them in the same location as your residence, as long as you have some legal address for service of legal papers. And there is an Escapee RV park in Pharump. Of course, they don't have mail forwarding there.
If all that you seek is a mail forwarding service that you can use as your legal address to be a Nevada resident, there are several that advertise in the RV magazines such as Motorhome and Trailer Life. I know there is one in Henderson.
I use the Postal Place downtown on 4th Street (Vegas). They forward mail every Saturday, don't know whether that's often enough for you. You just rent a box there and leave a deposit for forwarding postage, they charge actual postage costs against that. Price is reasonable, but I can't remember right off hand how much the box costs.
We use UPS store for mail forwarding if that's all you want. They are EVERYWHERE. We use the one in Rapid City, SD and they will send on a regular basis or whenever you call them, plus there is another one similar the Escapees, Alternative Resources in South Dakota. I believe they advertise in TL and MH. Can't remember the name of the town.
Dont think that UPS is an economical way especially since they announced today that they are putting a surcharge on ground deliveries. We have always had great luck with Good Sam in Pensacola. Pick a town (small) somewhere on your itinerary, call Good Sam and tell them when and where and it will be waiting for you c/o General Delivery. Good Sam did try UPS last year for a short period and abandoned it due to high cost to the membership.