You should be able to install just about any fan you want. Opening should be, or is pre-wired. You'll be looking for two unconnected wires, black and white or red and white and their probably tucked up behind the flashing or moulding. Just take the interior trim piece off and dig a bit.
I installed two Maxxair Maxxfan Deluxe fans in our Lexi, bedroom and livingroom, and they do make a big difference. 10 speed in or out, with a remote. Easy install, about an hour. Get yourself a tube of Dicor Self Leveling Caulk, and a roll of butyl tape. Everything else is included.
And whatever brand you choose, check Amazon for it. They have just about the cheapest prices on all the brands. One's we got were almost $100 cheaper than Camping World.