I've stayed several times at the Fort Stockton smallMart as well as the NM Welcome Center. I wouldn't stop anywhere in El Paso.
Benson AZ is probably a better place to stop just ahead of Tucson. There's a large Walmart with their own parking lot.
Immediately north of Tucson, there's also a Walmart right off I-10 in Marana at Cortaro Road. I've seen a few RV's parked but it's a shared parking lot with Kohls and a couple of strip malls. Those are usually the WM's which don't allow overnights...
There's also a Cracker Barrel at the same interchange where you're more likely to be left alone.