I see your not getting any help yet.
Can you drop a vent surround and peek into the attic and measure ?
Vents would be on the side of ceiling frame , A/C will be mounted to 2 frames. Always found the layout of the frames easy to figure from on top of the roof. But I've only had a rubber roof on my RVs.
99DSDP 3884, Freightliner, XC, CAT 3126B, 300 HP /ALLISON 3060
2000 Caravan toad, Remco & Blue Ox.