This is the updated timeline according to the family:
Note they left Mt. Shasta at about noon and then were seen in Stagecoach at 3:48pm, not 2:30.
Mt Shasta to the Nellis AFB area in Las Vegas is an 11 hour drive, without stopping. I assume this is where they wanted to stop:
Desert Eagle RV Park - Nellis Life
There is also a huge off-limits Nellis Air Target and Target Practice area that is just outside of Tonopah. It's not an Air Base, but there is a chance that was the location they had entered and thought they were driving to. That could have been estimated at an 8 hour drive. That really does alight better with their trip progress, until they disappeared.
Were either Ronnie or Beverly ex-military?