Kartchner Caverns (Kartchner State Park) in Benson, AZ (east of Tucson, AZ,
I-10, to Hwy 90 south toward Sierra Vista) is an awesome 'living' cave and the tours are great! The formations are continuing to grow as opposed to Carlsbad (which are great, too, but in a different way). Both are advisable. Kartchner also has a very nice campground with electric and dump station.
Your route from Yuma to ElPaso will be relatively flat. Just before ElPaso we always take the 375 Loop (Trans Mountain Rd.) over toward Guadalupe Nat'l Park (no hookups at their campground & it will be very hot) and on to Carlsbad. For Carlsbad we stay at Brantley Lake State Park which has electric. Hwy 375 is a steep uphill but RVers do it all the time. Have a good trip but it will be HOT all the way.