Dave, from another thread, I believe you recently acquired this trailer with roof AC already installed by a PO, but possibly not reinforced, correct? I also recall that it had a cut-out near the door where a closet AC may have been previously installed (which was the normal way back in 1979). I couldn't tell for sure from the pictures whether it had a permanent
fiberglass patch or something temporarily fixed over the opening.
You have two options, as I see it.
(1). Keep the roof AC and add reinforcement. I believe the usual way is to add two ribs across the trolley part of the roof fore and aft of the AC unit. If you do a Google site search you should be able to pull up some details. ADV: you already have a (presumably) working unit, better distribution of cooled air, less work. DIS: reduced headroom, towing aerodynamics, aesthetics of add-on bracing, vibration.
(2). Remove the roof AC and reinstall a garden variety window unit in the old closet location. I think - I'm not sure - the hole in the roof for the AC is a standard 14"x14" so you could replace it with a DC-powered vent (Fantastic Fan or similar) for times when you don't have shore power. ADV: bracing not needed, headroom, factory appearance, window AC much cheaper when time to replace. DIS: less efficient air circulation, have to buy new unit & sell old, more work.
I'm thinking your decision will depend on how the old hole was patched, where the AC wires are now, the condition of the roof unit, how much work you want to do, how much hot/humid weather camping you do, and whether you want an "original" appearance.