How do the conversion companies make the ceilings of trailers flat instead of curved?  Then, how do you work around the A/C this is already installed?

If you mechanically attach wood stringers to the metal ribs, you can make any ceiling profile you want, flat, recessed, curved, multilevel etc. The area around the air conditioner is boxed similarly to a window frame. The spaces created between the bottom edge of the stringers and the roof surface, allow the installation of recessed lighting, wiring, switches and insulation.

After the framework is completed, the final ceiling panels are attached to the wood stringers, using fasteners or various construction adhesives. Trim pieces are added to cover the fasteners or gaps in the panels.

BOL   Gard

If you look at the top rail on you trailer(the round part on the sides) inside towards the ceiling there will be a flat section(sometimes and Inch sometimes less), I take firring strips 1x3's or 1x4's and run across the trailer, to support the center, I measure down from the aluminum cross braces and space that down with 1x3"s from the front to the rear and screw the cross board to it. If that gap is very big, say 4" or more then I support the cross braces with wire, kind of like a drop ceiling in a house. I use sheets of Luan for the actual ceiling.