I did something similar, bought a battery operated or 12v camping fan at walmart, took the back part off, then replaced the 4 screws in the vent that hold the trim piece up with for lack of a better term-snap screws in the rv. I then mounted the fan on a 14x14 cutout, put the other part of the snap screws on the board and bought a flexible 20watt solar panel and taped it down with eternabond tape. I used a quick disconnect connector and now can snap that fan into the vent to suck the hot air out. That thing can really get cranking.
I did that in our bedroom. In the main part of the fiver I bought a vortex fan that fits right into the vent and put a 10watt solar panel on that too.
I just did this stuff because it gets so hot while sitting there in the storage lot.