It's almost summer, and you drove across a desert in what is essentially a van with a metal floor and maybe a half inch of rubber on it, with the engine and the transmission shedding heat in the airstream under the bus, on a platform that is more than 20 years old, and probably didn't have a large enough A/C system for the cubic volume in the first place.... And you're wondering if it being "hot" is normal? Lol, forgive me, but that is pretty funny.
I would say that you're probably alright, and that you're just stressing the limits of what that old bird can do.
Maybe give her an extra break somewhere out of the heat, and let things cool down a bit. Besides, part of traveling is enjoying the ride, and that actually means stopping and getting out of the car and meeting some new and interesting people and seeing some interesting places along the way, sometimes a bit more off the beaten trail. Stop and see a wax museum or that "world's largest ball of yarn" or whatever it is, and maybe get an interesting photograph or two. And even if it sucks completely, it's not like you're signing up to have and to hold... Check your fluids, top off your tank, buy a drink, and keep on trucking to the next spot.
Speed will kill you, yes, but it will also kill your motor and tranny. A lot of the van-cutaways are using the stock one-ton drivetrain, and maybe have an upgraded cooling system--but maybe not. Sure, there's more weight, but the real thing that is usually making the engine work overtime is the wind-resistance because of the <ahem> "streamlining" (or lack thereof). Keep the speedo around 65 or less, and turn off the OD when you're going up hills. Make sure that you're not leaking or burning coolant, and you should be fine.
When you get to building it out, you should be able to add some insulation to keep the heat from creeping up inside the compartment.