Draw a floorplan using graph paper. Decide how many people your camper will sleep and what features you will include, including kitchen items, storage and entertainment areas. Map out the configurations on paper, making sure the features are a proportionate size. Get ideas from other RV building sites like Butler Projects or Cheap RV Living.
Cut holes for windows, vents and a door if necessary. Since your trailer was not designed to have people sleeping in it, you'll need to make changes to make it safe. Before you add any other features, purchase several windows from an RV supplier or an RV salvage yard. Use a saber saw or jigsaw to cut the holes where they'll go, but hold off on installing them until you have some other features installed.
Build simple wall and floor framing, and install insulation. Since your trailer was not designed for human habitation, you'll likely need to put some insulation in it to keep it from getting too cold in the winter and hot in the summer. Use 2-by-2s to build a simple frame on the walls and floor of your trailer, then place a thin layer of insulation in between the frame pieces.
Wire the trailer for electrical features and plumbing. Consult a professional for this step if you are not confident in your skills.
Install paneling and flooring. Once you have your framing and insulation in place, cover the walls with a lightweight paneling made of fiberglass, vinyl or fiberboard. Cover the floor frame with plywood, and then finish it off with self-stick vinyl tiles, carpeting, wood, or any other flooring you desire.
Install windows, venting and the door (if you don't already have a door you want to use). Caulk around each feature to ensure it keeps out moisture and wind.
Install interior features. Get creative. Troll an RV salvage yard to find used tables, counters, closets, and whatever else you need.
