As Floyd has posted putting in low profile rafters would be important as would lining them up with inside cabinets and or supports to carry the load down to the floor. Allot more work than just cutting a hole and plopping the air conditioner on the roof. You would also look into the relationship of the weight
over the axle
and how you will get power up to the air conditioner unit. This single task gets into the whole trailer package. Interior, exterior, structural, electrical
(existing and new)
Remember when you mold in your structural support for the air conditioner that you prop up the roof and correct any sagging that may have creeped in over time or sagging by added weight so you don't have low puddle spots around the air conditioner just waiting to leak.