Enclosed cargo trailers are often large, unobstructed spaces which lend themselves to conversion into home-made towable campers. They have strong, rigid frames and bodywork, typically have roof ventilation vents preinstalled, and many have a side personnel door toward the front. The basic provisions a camper must afford are shelter from the elements, security, a supply of electricity and water, a galley and a bathroom and sleeping accommodations. Comprehensive toolkits, a broad knowledge of construction principles and an extensive skillset will be necessary to carry out this project.
Insulate the trailer by cutting down eight-by-four-foot rigid panels of thermal insulation to sizes that fit between the support hoops and lateral rails of the trailer, then gluing them in place. Use panels with an impermeable foil liner on one side to prevent condensation forming between the layers.
Line the interior with lightweight panelling by first cutting the sheets of panelling to size outside the trailer, then moving them in and attaching them to the support hoops and lateral rails using self-tapping machine screws and cup washers.
Fit such furniture as will suit your purposes. Collapsible bunk beds, which are hinged midway up the walls and fold flat, are convenient space savers. Kitchen units are available from home improvement warehouses, which come complete with sinks, plumbing, cabinet doors and fastener hardware. Most large superstores sell camping stoves and water heaters. Cassette toilet/shower combos can be sourced from most camping or recreational supply outlets, and are easy to make private by simply hanging a curtain and curtain rail.
Install a 12-volt electrical supply that incorporates at least lighting and some outlets for appliances or entertainment equipment. Run the supply from a deep cycle marine or RV battery, which can be charged through the trailer hitch from the tow vehicle while under way. The 12-volt circuits must always complete, meaning the supply leaves the battery's positive terminal, passes through the appliance, and returns to the battery's negative terminal.
Rig up a basic water supply incorporating a freshwater holding tank, a 12-volt on-demand pump, and runs of pipe to your sink, water heater and shower as required. A holding tank will be necessary under the trailer for grey water to exit into, and some form of operable shut-off valve to facilitate evacuation at legal dump stations.
The cost of a trailer conversion is unlikely to be recouped at sale. If storage considerations allow, it usually makes more financial sense to simply buy a travel trailer with the conversion budget.