Carlsbad Caverns would be quite a long detour off your I-40 route.
I prefer staying inside GCNP because sunrise and sunset times are beautiful on the canyon walls. Plus I'm on my own schedule. Must get campsite reservations.
I agree, you won't enjoy driving RV in southern CA.
Use this website to find interesting or quirky places along the way. Click on Maps and find your route in each State. Pass your mouse over all the pinpoints.
Roadside AmericaWe found the Oklahoma City Memorial to be hauntingly beautiful (we only visited outside portion). Stop at Tucumcari, NM for some Route 66 kitsch.