I hear you on that, Kyle. I've heard the Burros and U-hauls described as "double hulled," but to my mind the inside is more of a molded liner -- probably used because it is easy to put in and keep clean (not that those are bad qualities, mind you). It does attach to the shell in some places, so I'm sure it provides some support and "doubling," but certainly not as much as it could.
Also, you are right to be thinking about stress risers, point loads, etc. that you could be creating by adding stiffeners. You don't want to cure to be worse than the cause.
It's hard for me to recommend anything specific without being there to poke and prod (the trailer), but here are some thoughts, coming from experience with boats:
Actually, wait, let me back up a bit. I just want to describe how, in fiberglass boat-building, there are a few ways to provide stiffness and support (because fiberglass is inherently quite floppy). With a fiberglass boat, or camper, you want a monocoque structure, wherein the skin and the stiffness work as structureal members.
Typically in the earlier days (1960s, early 70s), The interior of a boat was "stick built" Support for the hull was given by bulkheads (transverse interior walls) that were bonded to the hull and deck, usually with fiberglass tabbing (like tape strips). Also, stringers were used (longitudinal hat-shaped supports fiberglassed to the hull).
Then in the 80s or so, builders started using pre-formed "liners." These were usually molded fiberglass, just like a shower stall or a Burro
liner. They saved money (skilled labor), and time (just drop it in and you are basically done; no carpentry needed). Also, builders started using structural grids under the liners (I'm not an expert on this, but I think I have the gist). These are molded fiberglass grids (say, half pipe shapes laid out like a Tic-Tac-Toe board) that are bonded in under the liner. They provide support for the boat in combination with bulkheads (which are now often simply slotted into a groove in the liner, not tabbed). But the liner and the structural grid are not always together; you can have one without the other, such as in the case of a U-haul
(at least I have never seen evidence of any structural grid).
For the deck of a fiberglass boat, usually some kind of coring/sandwich is used. Essentially, two fiberglass skins are held apart by a core material, and the tension of the skins (like an I-beam) make it strong and stiff.
So okay, where was I?
Oh yes, some thoughts about how to strengthen:
1) Core an area (like a boat deck). This would involve sticking some kind of thin core to the shell (1/4" balsa, foam, thin ply) and then fiberglassing over it. You could perhaps taper the core to make a transition.
2) Tab in a bulkhead. When boat bulkheads are installed they are intentionally left 1/2" or so shy of the hull. If they fit perfectly they would make a hard spot that would show as a bulge. The gap is often filled with closed cell foam, then fillets are made (tapered corners of something like thickened epoxy; or you can taper the foam in a trapezoid shape), and then fiberglass tabbing is applied on boat sides. The tabbing spreads the load over a wide area, and makes a solid connection. Flexing is the enemy.
3) Tab in cabinetry. In the case of a Boler
type, one could tab the cabinetry in (similar to bulkheads, above) and eliminate the rivets while making a stronger shell (some people think the rivets are there to allow for flexing; but flex is the enemy and the rivets are there because they are economical to install in a production setting, and our campers had to be economically viable).
4) Stringers. Stringers are longitudinal stiffeners. They were originally wood (as were boats!), but with fiberglass they can be wood, hollow plastic, half pipe, foam... the point is to cover them with fiberglass that extends out onto the hull. It's the shape that provides the strength, typically (althoughthe wood core can be engineered to be part of it, I would say not as a rule). A stringer is pretty favorable for not making hard spots yet providing support.
So, on your cabinets, you could probably tab them to the hull on the inside (where it would not show and would not have to be overly sleek). You would just want to ascertain that you would not be creating any hard spots, or transferring a load to a component that could not take it (imagine the cabinets pulling the roof down). Now, that is not likely - after all Boler/clones are not super heavy duty, and the cabinets are riveted to the roof, but it is still something to think about (and Bolers/clones do have a "wrought iron" support from the inside edge of the upper cabinet down to the counter.
I would just say to think about where the loads and stresses will go (as you have been doing), and think about spreading any point loads. A nice wide set of tabs might very well give you a stronger more rigid overall structure (because although point loads are no good, overall rigidity is good).
Okay... still awake?
Of course that doesn't mean you can't ask more specific questions once you have sussed it out a bit more
And as always, more photos = more better