Mount a bathtub on a metal stand about a foot high. Fill the tub with water. Submerge a coil of copper tubing in the tub with one end of the tubing connected to a water spigot and the other end to a shower head inside a trailer. Build a fire under the tub. The heated water in the tub will warm the water in the tubing. Open the spigot and, voilá! Hot water flowing to your shower.