Map out the floor plan that you have chosen either from an existing set of plans or your own custom plans on graph paper. Look at a few layouts and plans to get space-saving ideas for your camper. Putting fold-up bunk beds and couches that convert into beds are some ideas for space saving in your camper plans.
Cut the holes for the windows, vents and doors in the places mapped out from your floor plan. Make sure you purchase the windows and doors first so you know the exact size to cut the holes in the utility trailer. Purchase the windows, vents and doors at an RV supply store or salvage yard.
Build out the walls, ceiling and floor with 2" x 2" framing boards and add insulation. Since the utility trailer was not built to have people sleeping in it, it is not insulated and needs to be.
Install wiring and plumbing. The camp trailer will need lights, heat, plug-ins and wiring for appliances and electronics. Plumbing will need to be installed for faucets, toilets and waste water. A waste water tank will also be needed.
Install flooring and walls. After laying the insulation on the floor, cover with plywood or particle board made for sub-flooring. Add vinyl or carpet of choice. Install walls and ceiling using paneling, cutting to allow for doors, windows and vents you previously cut.
Hang doors and install windows and vents. Make sure that each is properly sealed to prevent moisture and air from passing through. Use sealants and weather stripping for sealing these areas.
Install the rest of the camp trailer features following the plans that you mapped out in the beginning. This includes furniture, appliances, walls, lighting, cabinetry, counters and plumbing fixtures like toilets, sinks and showers. Many of these items can be found at salvage yards if you are looking for inexpensive alternatives. You can make some of these features or hire a professional to custom make things like cabinets.