Hey guys,
Don't forget the Webasto Diesel Fired auxiliary hot water heater. This is used on the latest modern trucks so you should be able to find em in the truck junkyard. Here is the link.
http://www.webasto.us/press/en/am_trucks_heaters_823.html
Here are the specs.
Specifications BlueHeat Thermo Top C
Weight (kg) 3.2
Max. heating duration (Minutes) 60
Heat output (kW) full/partial 5.2/2.6
Fuel consumption (kg/h) max. per full heating phase appr. 0.3 / 0.6
Power consumption (W) full/partial 32/22
Variable Heat Output 8,600 to 17,200 Btu/h
Parking ventilation (optional) +
Dimensions (mm; LxWxH) 214x106x168
Controls
Thermo Top C
24 Hour, 3 Event Digital Timer
or
Remote control
And they are available in 12 or 24 volt if you want to buy new.
Hotboxes are good but this is another option.
Food for thought.