And to expand on it a bit more, what you are really trying to prevent is a change of shape. That is, the whole trailer getting wider and shorter. It's like a house in that there is more than one way to skin a cat (collar ties/rafter vs. structural ridge vs. etc.).
I think the closet does a pretty good job of keeping the shape on the door side (it could be better, of course). The upper cabinet/metal brace/lower cabinet support on the kitchen side is not quite as good at what it does, to my mind. There is room for improvement.
You could simply narrow the closet and kitchen by 6" and keep the supports as they are (but 6" further forward).
A couple of other ideas:
1) For the kitchen side, you could tab in a plywood "boomerang" to the wall (not quite boomerang shape, but hopefully that gives you the idea). The point there is that if the walls can't bow out, the roof can't sag down (unless there is a very heavy point load that sags the middle of the roof). This would be much stronger than what is there now, and could be tucked back a bit more toward the wall.
2) You could tab in a structural arch on the ceiling; say, where the bed meets the kitchen/closet.
3) You could make your "desk" open on only one side (say, toward the bed) and the front, and then make sure the wall on the other side is even more structural than it is.
Basically, if the roof can't sag, the walls can't bow out; and if the walls can't bow out, the roof can't sag. So you just need something that prevents either/or. Even light
plywood (or other suitable material), if tabbed to the walls or ceiling, would be quite strong and reinforcing.
The thing is that fiberglass is actually a fairly "floppy" material. So you just have to account for that. A fiberglass boat is the same way. It's a mixture of "bulkheads," which are like walls tabbed to the hull, and partial bulkheads or (sort of) "knees" which are also tabbed to the hull but which give more openness. One difference is that on a boat you are trying to make the panels really strong even in the areas between supports*; whereas our campers aren't really heavy or strong enough for that, as built. The goal seems to have been lightness, affordability, and the ability to keep the basic shape; but not necessarily the ability to stand up to point loading.
I hope that made sense!
*Boats also have "stringers," which are like long shapes (think half circle) that run the length of the hull and are tabbed to it (you can literally use foam or hollow half rounds of plastic). The give quite a bit of "shape strength" to a hull, and help it to resist deformation ("oil canning") when plowing through the waves.