So presumably, by "use the RV" BT may have wanted to take a hot shower, or recline in one of those comfy chairs and watch her favorite novella. MOO
Obviously, she would be seeking air conditioning (available in the truck) and water (available in the 5th wheel, which also could have had A/C - I'm assuming it does), and perhaps some electrolytes. Just getting out of direct sun at 2:30 would appeal to most people (but very little shade near the RV, that's for sure).
I can't imagine wanting a hot shower, only a cold one. It's cooled off a little here in SoCal, but it was 105F by 4 pm when Barb disappeared. By the time it's 90F in our yard, we have turned our water heater down significantly.
As to camping:
We know of no reservations that they had made, but...really...they didn't need reserations. NO campground in the Mojave Preserve or in Joshua Tree takes reservations. There were no reservation-taking campgrounds in the direction they were going until...you get near Vegas. But no one needs a reservation in Vegas, either. There are over a thousand casino-based spots, they never fill up, especially not in July. Heck, they could have gone to Laughlin to camp, there are 750 spots, most of them open, at Paradise.
We don't know where they were going next.
But I think it's probably that they may have discussed going to a campground some 15 miles north of where they were. It was not full, and it almost never gets full in summer. No hook-ups. Higher elevation than Death Valley's main area, A/C would be very very desirable, night time temps were probably close to 90 or the high 80's (it's been a fairly cool summer).
Most 5th wheels have rooftop A/C and come with a built-in generator. To get anything else, living in BHCity would be really weird, especially as the additional cost is not that much compared to everything else they had just purchased (the truck was not cheap). It's really hard to find a 5th wheel on a lot that doesn't have A/C. For dog owners, it's a super cool way to travel.