Cool. Good part with the solar panels is that the panels themselves can put your roof in the shade, so some strategic placement of them might help, say over your bed. Or whatever other portion of the ceiling you want to keep cooler.
Insulation, like mylar / radiant barrier -coated 2" rigid foam, would help, but it will also heat and retain that heat for as long as it took to get it in there. Keeping the sun off the vehicle skin from the outside would work better. But then again the insulation will come in handy in the winter months. Especially in the deserts.
Had another idea for the fans, a pop up vent like the sort of roof-mounted driving lights on some baja racers. Down position, the hatch flap seals shut on the roof. Up position, that flap is vertical and the hatch is sealed from the inside by a same-sized panel filled with your fans. Same draw latch on the inside ceiling could hold the L-shaped fixture in either position. Some appropriate rubber gasket / seal on both faces of the opening in the roof. Rig it right and have the fan up position close the circuit and activate the fans. Simply opening the fans into position turns them on.
as an aside, I've been sketching out a set of home-made silnylon awnings for my Suburban project, a large pair of trapezoid-shaped awnings that can be arranged on both sides of the vehicle, or at right angles around the side and rear of the vehicle, or put together off the same side as a larger hexagonal shape. Along with them, I want to make a vehicle shade that is deployed over the top of the vehicle from the front corners of the grille guard all the way back to the rear roller on the roof rack. Suspended shade for the vehicle itself. And it can be doubled over and used as a travel cover for stuff stored on the rack when on the move. It's what led to my suggestion for the 'trampoline' idea.
eta - I also just recently finished insulating my aluminum garage door with mylar-coated 1" rigid foam insulation, real easy project and the easiest least messy way to work with the foam was using a long serrated carving knife, like you'd use for carving a turkey. Goes thru styrofoam like butter on the draw stroke, very little mess. Stay away from sawblades, they just tear tiny bits of material out and cast it all over. The knife cleaves.