I actually really love and appreciate this feature of having a pop up camper! If you put the top up, open the windows, and turn on the fan I've been able to leave my dog in the camper very comfortably even when it's over 90 outside no problem. I have a thermostat inside so can check temps and make sure. It's been so awesome to have an option to leave the dog in the rig when it's too hot to leave him in the cab of the truck!
A few things to consider -
1) In my experience, the fan has been crucial (set to blowing inside to outside, not outside to inside),
2) Also critical is making sure to keep any windows that are in sun closed, closing curtains for sun-side windows, and I've put a towel or otherwise covered up the door window as well if there's sun from that side
3) If it was cool or cold where you camped the night before, and you buttoned up the camper in the morning before it got hot out, odds are your camper is actually much colder than the outside temps which buys you time while it heats up to ambient air (feel the floor when you put them in - it's probably pretty cool)
4) If it's 95 outside, your dog would survive laying in the shade next to the truck, so by the same logic they will survive it being 95 with a breeze (i.e. the fan) laying in the shade inside the camper - any exertion, higher temps, less airflow, or direct sun changes that equation
Those are my two cents! I've left my dog plenty in the camper, top up (usually just one side), fan on, shady-side windows down, with water, and he's been 100% fine and he has terrible heat tolerance (black german shepherd mix who gets hot at like 65). He's not even panting when I come back!