Semi trailers are built to haul 30 tons or so of cargo for hundreds of thousands of miles during its life. The older spring style suspensions were set for maximum load, Empty they bounce down the highway like a paint shaker on steroids. I hated pulling empties back when i was driving. There is no way you are even going to come close to putting enough weight into the trailer when converting to an RV to cause the springs to flex enough to soften the ride. Wood construction, cabinets etc, are not going to live long in that environment. Probably not much issue for a fixed location use such as Mudcat's or if infrequently moved at low speeds and short distances.
A more modern trailer with adjustable air suspension will help greatly, especially in conjunction with rigid spray foam insulation, but only if being pulled with a tractor with air suspension also. I would glue and screw all my connections, and use glue on any plywood sheathing to help resist shear movement. Also seek out or build quality cabinets made from plywood, not transplants from an RV or Box Store chipboard cabinets.
Structurally trailers have quite a bit of torsional flex due to being a long empty tube, with a big hole in the end to load cargo. During normal use, cargo is loaded loose into the trailer any twisting that occurs during transport does not really get transmitted into the cargo causing any damage. When you start adding walls and securing cabinets to the trailer walls, that twisting and flexing transmits shear forces directly into those objects, i.e. it will try to rack the wall back and forth causing fasteners to work loose and wall paneling to buckle. The only way to reduce that effect, is to either allow some flexibility in the joints from trailer to interior walls using expansion joints which is difficult at best, or sufficiently brace the wall to resist the shear and reduce the amount the trailer flexes. Gluing and screwing plywood to ea. side of the wall under the finish is one way. Each interior wall added divides the trailer into smaller structural 'boxes' with the end result of each interior wall only having to carry portion of the total shear loading.
Lastly make sure you are aware of any issues with using a semi trailer as an RV in states you plan to travel to. Some states will not allow an RV with a semi style 5th wheel, some have overall length restrictions for RV combinations etc. of course if you plan on using a CDL and running under DOT regulations, then that's a different story.
Dave
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