Abstract: A unified floor frame assembly has two elongate outer load supporting beams formed of elongate beam sections that are butt-spliced to be cambered in parallel vertical planes to counter forces that may tend to cause sagging of the floor frame assembly during transportation. At inner vertical perimeter surfaces of the elongate beams are provided attachment plates for attachment, first, of a wheel carrier assembly detachably mountable thereto with a plurality of wheels partially recessed within the floor frame assembly and, second, a towing hitch assembly attachable to a forward end of the floor frame assembly for applying a towing force thereat. A moisture, dirt, insect and pest excluding thin covering is provided underneath the floor frame assembly and sections of heating and ventilating ducting, piping, wiring and the like are includable during manufacture of the floor frame assembly.