...have you ever thought of a full-spectrum reflecting solar "tarp" that you could drape over the roof of your truck camper?? These things are used to cover anything from entire industrial building roofs in very heat-intensive climates (Arizona, Middle East, etc), to house roofs (to substantially reduce cooling bills where roof insulation is often not nearly sufficient, or almost entirely absent), to garden sheds, to camping beach tarps. One that comes to mind is: Sola Reflexx cool-tarp (among others). These are designed to reflect the full spectrum of in-coming solar...
I suppose that you could buy one of these tarps, and cut out the dimension required to insert into a camper roof vent (and, leave the vent lid open about 1/8th of an inch, to protect, or, as insurance against the Heki potentially self-destructing from incoming solar).
Good luck,
S-