If you have an aluminum checker plate box, it should be OK. Black box, not so good.
I have an aluminum checker plate box in the back of my Ranger, and things stay pretty cool because the sun's heat is reflected away.
A black box will absorb heat like crazy, and turn into an oven real quick.
Another thing to think about is the fuel vapours coming out of the tanks as things heat up. Just be careful with anything that can make a spark when you open up after a hot day.
Happy Glamping.
__________________
Happy Glamping, Norman & Elna. 2008 Winnebago Adventurer 38J, W24, dozens of small thirsty ponies. Retired after 40 years wrenching on trucks! 2010 Ford Ranger toad with bicycles or KLR 650 in the back.
Easy to spot an RVer, they always walk around with a screwdriver or wrench in one hand!