On the way to Carlsbad be sure to go down US-380 / US-54 and visit Valley of Fires BLM campground. It has a great nature walk across the top of the lava flow.
30 Amp power with water. Great shower house. Dump station.
Valley of FiresAnother good stop is Three Rivers Petroglyphs. It has two 50 amp with water sites. Nice walk through the rocks and cool markings on the rocks.
Three Rivers PetroglyphsI've never been there - but want to visit Bottomless Lakes SP just outside Roswell
Bottomless LakesComing back into Texas two great stops are Monahans Sandhills SP and Balmorhea SP.
While you can drive through the sand dunes at White Sands - you can't camp there in an RV. You can at Monahans - hopefully you won't get too much sand in the rig.
Monahans Sandhills SPBalmorhea is simply the very best swimming hole in the entire Texas State Parks system.
Balmorhea SP