Since there seem to be a few questions about this and not a lot of real world experience I thought I would address this.
In my 26 foot Hybrid I have two standard roof vents. The first thing I did was follow the crowd and get those white plastic vent covers that allow you to leave the vents open all the time. I wanted this because closed up campers tend to sweat and smell a lot.
I was not pleased with the results because these vents also blocked a lot of air flow when camping and the air still seemed to stagnate. I then installed an 80 MM computer fan in each roof vent.
I can honestly tell you that this made a huge difference! The heat level and that adhesive smell common to newer camper interiors was reduced drastically and there is only a slight hum instead of the full on racket of the fantastic fan the is in the bathroom of this same trailer.
I figure I have about $10 into the project and it uses an almost negligible amount of battery when we are dry camping.
My tear will have a standard $18 roof vent with 2 LED 80MM computer fans (free from scrapped out computer systems). These fans will provide both air movement that can be noticed and a nice glowing light in the same small package. They are quiet and efficient for both intended purposes.
The fan and light functions will have separate switches.
Yes! There will be pictures and if you like I could take some pictures of the existing setup in my big camper.