A lot of that depends on how the walls of your trailer are constructed. Are they a single skin of sheet aluminum riveted/screwed to a metal internal frame? Or, are the walls a sandwich construction?
If it's just a skin and your chosen mounting location is on the sheet metal, then I'd recommend through bolts, with a SS screw, and SS washer on the outside, with a dab of clear silicone under the washer and screw head when you tighten it. If you can, find a metal to metal glue for the flange on the inside for the sheer, and then the SS screws are just working in tension. You could probably use a caulk with an adhesive quality to it as well.
If the inner 'skeleton' has ribs that are close enough, you could also run some flat stock, angle, channel or rectangular tube between the ribs, and attach your cabinets to them.
If it's a sandwich type wall, that's a whole different story. You really need to make sure you don't crush the sandwich (you'll really affect the strength if you do) and likely have to rely on the 'glue' to do a lot of the work for you, and keep the through bolts to only areas with a stiffening rib inside the sandwich.
PE
