We make that trip every year going out and coming back from summer vacation. I would stay at Oliver Lee SP outside Alamogordo, but be sure to reserve one of the few water/electricity sites. Near "Alamo," White Sands National Monument is a really fascinating place to visit. If you do that, I'd spend a couple nights at Alamogordo.
I would NOT get to San Antonio by going through Roswell, Carlsbad, and Pecos Texas. The traffic between Carlsbad and Pecos can be especially unnerving. The oil traffic on those two-lane roads is heavy, and those trucks travel fast. We go south on US 54 from Alamogordo to 375 and take that around the east side of El Paso to IH-10 east. From there, just decide where you'd like to stop to shorten the trip. There are few nice RV parks between El Paso and Junction, though Fort Stockton has some parks that are good enough for an overnight...but they are crowded with oil workers. I'd advise going through to Junction and staying at
Junction North Llano River RV park if you want an easy overnight with full hookups and a great BBQ place nearby (Lum's BBQ). If you want natural beauty without full hookups (water/elec only), then stay at our favorite state park South Llano River SP.
From Junction it's a two-hour drive into San Antonio.
