IF you've got unlimited coin, the espar/webasto/wintermizer is the way to go. Used to run an old gas fired "HOTBOX" on my big truck, an old big cam 1 400 cummins. Much below 0c it needed help, only a 14.5/1 compression engine. THe hotbox would bring it up to show heat on the temp guage in about 15 minutes, and remember that engine is probably in the area of 3000 lbs. Had it hooked through the bunk heater too. Thermostat controlled the fan in the bunk, hotbox would heat the water and circulate it through the engine and the bunk at about 160 degrees. Had to put a bigger tank on it though, had a 5 gallon boat tank strapped down on the back deck.
Barring big bucks, everything listed by others is a big plus. In a pinch, a propane torch to warm the intake manifold, or even a big pail of hot water dumped over the engine might do the trick. Water can cause other problems though.........