The most basic of the alternative devices is a 12-volt bunk warmer, similar to an electric blanket. Add a pair of long johns for that extra bit of warmth. Today’s engines can start at temperatures as low as minus 20 degrees, so shutting off the engine is a viable option.
In hot weather, leaving windows open compromises security, but marine-style roof-mounted 12-volt exhaust fans or window fans with security screens are often effective.
Insulation is key to keeping a sleeper warm or cool. Even with custom-made sleepers, specify extra insulation. Heat loss in winter and heat gain in summer can be reduced dramatically, especially with some of the new spray-on or brush-on ceramic and metalized compounds.
Next on the scale of energy efficiency come fuel-fired heaters. While typical APUs burn 0.2 gallons per hour of diesel, a fuel-fired bunk heater can burn as little as one-fourth of that, 0.05 gallons per hour.
Because fuel-fired heaters are the most fuel-efficient cold weather devices, they may be paired with the warm weather efficient systems such as battery powered air conditioners or generators with electric air conditioners to give you the most efficient year-round comfort.