Last year I had to replace the vent lids (14" Ventline/Elixir style), on our Salem. I did notice that in both the bedroom and bathroom that their is wiring (fan, lights and A/C) and duct work up their. There was also a certain amount of structural reinforcement incorporated into the ceiling trusses for the vent mounting. I would be leery of cutting a hole in the ceiling without knowing what was where. Perhaps you could remove the bottom fascia of your current vents and have a look? Also, I don't know how adding aftermarket vents would affect your roof warranty.
'07 K3500 Silverado LT Crew Duramax (LBZ)
2016 Salem 27RKSS
1984 CHEV SCOTTSDALE K20 2GCGK24J0E1XXXXXX (Chevrolet Legends-Class of 2019)
"...exhaust fluid? We don't need no stinkin' exhaust fluid"