It might be helpful if you could peek up under the interior duct to see what provisions they made for mounting. In my U-Haul
VT16, they framed out the opening with 2x4s (cut down to about 2x3) and added some galvanized steel covering to protect the wood. The bolts that run up from the interior vent to the rooftop AC unit will try to squeeze things together, so it needs some strong support here!
(I no longer have roof AC, but a swamp cooler. The black wires you see going through the frame are DC power and water connections because I am using the headliner as a raceway for the roof mounted equipment) In the photo below I removed the vent cover, which mounts on light
studs, not the relatively large bolts used to hold a 100lb AC unit in place! You can see the galvanized plates, a hint of a 2x4, and you're looking up into the hood of the swamp cooler. That black circular thingy on the left top of the photo is the foam filter that humidifies the air. The fan spins in there and spews water out on the foam core of the filter. I don't think a swamp cooler would work very well in your climate, but I wanted you to know what you're seeing here!)