Mail trucks make great donor vehicles for conversion to homemade campers. The style of trucks used by the United States Postal Service and UPS are broad enough to accommodate a full-size bed, tall enough for most people to stand upright in, and already have access doors both from the cab and directly to the outside. While a do-it-yourself conversion is a complex, time-consuming and potentially costly project, you will end up with a custom level of provision designed specifically to serve your purposes. Before starting out on such an involved undertaking, a lot of careful planning and decision-making must be carried out.
Plan your 12 volt electrical system. The most primitive system involves a 12 volt deep cycle battery wired to a light fixture and a minimal number 12 volt outlets; 12 volt appliances such as a refrigerator, a heater, a stove, a water pump and entertainment equipment can then be added in the future. More complex circuits involve the installation of a fuse board, an alarm system and even a battery isolator to charge the deep cycle battery from the mail truck's alternator while under way.
Decide whether a 120 volt installation should be added. By fitting a shore power cord, connected through a circuit breaker board, you would be able to operate household equipment in the converted mail truck. A 30 amp installation can safely operate virtually every appliance a camper can physically accommodate except a twin air conditioner system; to have both units working simultaneously a 50 amp hookup is required. If adding a 120 volt installation, consider fitting a converter charger to automatically charge your 12 volt deep cycle battery when you plug in to shore power.
Determine the level of provision required of your water system. Self-contained installations use a freshwater storage tank, a 12 volt on-demand pump, and storage tanks for waste located beneath the mail truck. Just like a 120 volt electrical hookup, the plumbing can be connected to external provision through a city water hose to a tap and a flexible boot to a sewer port. Both sources use the same pipework in the mail truck. Cassette toilet/shower combinations are an option worth investigating, because they allow bathroom use without any plumbing being necessary.
Consider the standard of insulation dictated by the locations your will use your camper. Colder climates will require internal insulation; rigid sheets can be glued to the walls and painted, and rolls of insulation can be held in place by interior panelling which also serves to make the interior more attractive and easier to clean. Most mail trucks have preinstalled roof vents, but if yours does not it is important to fit at least one if the camper will be used in hot climates.
Ensure your mail truck camper conversion subscribes to legal change-of-use requirements. Your state may mandate that a change-of-use declaration be made to the Department of Motor Vehicles, and some require an inspection for safety and weight be made.
Unless you upgrade the brakes and suspension, the total weight of the converted mail truck, including all freight, driver and passengers, must remain below the truck's pre-conversion stated maximum.