What works best is to build a false floor on the trailer bed. Although this will make for a low ceiling, it can provide an adequate space for plumbing, electrical wiring and any cabling you might want for entertainment accessories. Don't think of it as a crawlspace under a house--just the height of a 2 x 4 turned on edge is plenty of space.
The same also it true for the walls and ceiling. Only instead of using 2 x 4 studs, use either 2 x 2 or even aluminum channel for framing. Aluminum is strong and will really impact the total weight savings.
Install plumbing for toilets, sinks and drains. Install under the trailer bed potable water and holding tanks. Plumbing can come from the potable water tank and to the waste tank. An RV hand pump or even splurge on an electric pump to move the water. Electrical wiring for lights, appliances and wall sockets can all be installed in the framing space.
Keep it light. The floor can be 1/4-inch plywood covered with linoleum or vinyl that's easier to clean than carpet. Ceilings and wall can be covered with thin veneer wood paneling, again very light but decorative. Do not forget, however, and in the space around the electrical and plumbing to add insulation before covering the walls and floors. This will keep the metal trailer from baking in the summer and freezing in winter.
Once the trailer is framed out and the interior is covered, you can decide on doors, windows, counters, seating, storage space and whether or not you can install water closet with a chemical toilet. RV supply dealers have window and door kits to fasten directly to openings cut in the trailer wall and frame. There are also available modular cabinet and furniture units. Finally, you can decide just how much of a galley (kitchen) you can make room to install. Again, camping stoves, sinks and refrigerators are all available that will run either on installed portable generator power or off the tow vehicle's electrical supply.
