Have you ever seen any other of your trailer or other models of your brand with a vent in the area you want to put one?? The roof "might be" framed for one already.
Initially Jayco told me that our model (only came with one vent in bath) wasn't framed for a bedroom vent, but on inspection and using a good stud finder, found out it was framed in for a standard 16" vent. Only needed to mark and cut inside and then drill corner holes before going on the roof to finish. Did need to tap into a hot light wire for a fan (Vortex) in the vent. Wasn't a big job, but measure, check, measure, check (!!!) before cutting.
If it isn't framed, it's pretty easy to work from a rafter and add cross beams to support the inside flange of the vent. Also, if ducted a/c, you should be able to stay away from the ductwork, most I've seen don't go down the middle of the roof, usually to one side of center.
Dicor or a high dollar tape (Eternabond) for sealing to roof material.
