I installed a fan in the plain vent that came with my camper. I bought a new whole vent unit, with fan, on Ebay. It was cheaper than what FWC charged and the fan I got was fancier (with a lot of speeds and a remote control). I didn't remove the vent from the roof -- FWC had it well sealed and I didn't want to mess with that -- I just installed the fan part of the unit I bought in the existing vent housing. The rest of the unit is left over, so I have a spare lid and crank.
The wires were right there, with crimp-type splices on them. The only problem I had was that FWC uses a black wire for ground and, on the Fan-Tastic, the black wire is +12V. (Hooking it up backwards didn't seem to hurt it!) I had to cut away some of the plastic housing for the wires, but not where it shows. And, I had to cut the slip-on connector off the wire to the switch that turns off the motor when the lid is closed and crimp a new one on after I put the wire through the switch hole.
A pretty easy install, all-in-all.
If your camper is not pretty new, the wires may not already be in place, which would make the installation harder. FWC now pre-wires all campers for all the options, but they haven't always done that.