is the same way. If the roof hasnt been "prepped" from the factory, it's not wise to install an A/C up there. The fiberglass is alot thicker on the roof in the area where the A/C is mounted. As a personal opinion for anyone reading this that may be purchasing a new trailer; I have the roof-mount A/C and would not be without it. It doesnt take up ANY interior space (like the closet-methods do) and I dont have to man-handle it installing it in a window at campgrounds.
So, for adding a roof-mount A/C, you can add "ribs" at the interior's top, but I HIGHLY suggest you'd hire a professional that could fit the "curvature" of the trailer as I would not be confident unless they ran the FULL width of the interior of the trailer. And only then would I agree with this if it was attached to the sides via "posts".
Now with that said, most of the time, this will have to be installed with more holes through the roof/trailer sides. There may be other ways. I agree with others and go an alternative route:
1) Floor A/C but adequate ventilation is CRITICAL for the exterior of the A/C or it will be SHORT-lived.
2) Quick-Remove window A/C. My Scamp
has a large back window that is perfect for this if I needed it. I would NOT run it down the road though with the A/C installed in the window because of the bounce. You can get $150 window A/C's that will cool that trailer fine.
3) Split Unit. I've seen mods where they mount part of it on the tongue of the trailer and of course you have the interior portion. Someone on this group has done that. Personally, this is overkill for a small FG trailer and also EXPENSIVE.