If this was my project I would not use plywood i would use a piece of foam about 3 feet long and as wide as the channel in the roof top. Trim it to fit very close to the sides of the channel in its width and at the ends of the 3 foot cut it at a bevel so it will look like a nice transition. On the side that will be bonded to the ceiling, bond a couples of layers of fiberglass accross the surface and up the sides. After that cures flip it over and do the other side. This makes a foam filled box beam. After that is done prep the ceiling and the surface of the box beam and epoxy bond this beam centered accross the hole in the ceiling for the airconditioner. It is very important that it bonds tightly to the sides of the inside channel to add stiffness to the trailer.
Use prop rods and a piece of plywood wrapped in a plastic bag under this box beam from the floor to this box beam. Use enough force to lift any sag and and hold the beam until epoxy cures. Most epoxy doesn't reach full cure for at least 24 hours. I would leave it a couple of days.
Now I would turn to the top and cut the hole for the air conditioner in your new box beam.
Now you have to do pretty much the same thing for the top to make a level pad for the airconditioner due to the ridge of your top seam.
Leave the prop rods and plywood supporting the ceiling while you work on the top.
Cut the foam to fit over the top seam. I would not remove the bonded ridge. Coat the foam with resin and glass fabric just like you did on the bottom box beam. Then after cure you bond it to the top of the trailer. Stack some weight on it until cure. Now you have to do some bodywork to blend the airconditioning pad to the top of the trailer. Then cut the hole for the air conditioner and I would resin and glass fiber between the 2 holes.
Then you get to make the age old decision to re-gelcoat or paint
to blend the body work.
dont forget you have to run power into the airconditioner so you will have to make allowences for that.
This work is very messy so mask well and plan on dripping epoxy all over the place before your done. Even this may not be enough reinforcement or spread the load out far enough but it is as far as you can go without messing up the gellcoat in the pretty parts of the ceiling.
I am not an engineer and do not profess to be one. This is only the approach I would take if this was my personal trailer.