Here's what I would do. I'm guessing you meant 189FBS. I would search for pictures and online ads that show location of a vent that matches your floorpan. Maybe someone in this forum can assist. I'm not totally familiar with the 189FBS (I have a 210UDS). I thought there is/was one over the bed? I've attached a screenshot I found on youtube. It shows a vent inline with the light fixture over the bed area? If accurate, I would then drill/cut a hole through the ceiling first (not the roof). This would allow you to see inside and gradually enlarge the hole to center and/or avoid wiring and such. Then, and only then, when the size and location has been determined from inside, would I breach the roof. OR-if you don't have an AC unit, I'll bet there's a big free opening in the ceiling area where the factory would've put the AC?
2012 FunFinder X 210UDS
4-68 watt flexible panels; 1-215 glass panel/4 Battle Born LiFePO4 Deep Cycle Batteries
2014 Ford F350 6.7 Liter PowerStroke V-8 Diesel longbed, SRW