I just did this to a 2016 Coachmen Apex 239RBS. I had the same issue of not enough ventilation in the front (bed area) of the camper. I do have a fan (small, one speed) at the rear of the camper in the bathroom and thought about just replacing that with the Fantastik fan, but I just knew it would not work as well as having the fan up front.
My roof is nothing more than a sandwich of rubber over 1/4" luan plywood, 4" to 5" of styrofoam and the ceiling which is 1/4" luan. I used a magnet to locate supports (mine are steel) and they are 4' on center.
I have ceiling lights that run in the center and length of the camper that I was able to get power from. The wiring actually lays on the ceiling in a hollowed out "raceway" in the styrofoam.
They sell a vent install kit which will have butyl tape to use under the vent lip, mounting screws and dicor caulk to seal the top.
So far so good. No leaks and nice air movement up front.