The freeze thaw cycle causes movement of a concrete footer unless it is below the depth where soil will freeze. Because concrete is immobile it is subject to cracking(cant flex with the changing temps.) Sounds like you dont wnt to talk about trailers anymore but just to say, mine has 2x4 walls spaced at 16 in. on center, standard house insulation, 2x8 floor joists. Doors, windows and plumbing stuff can be harder to get. Just go to a mobile home dealer or order online. they are not all built alike. Im done.