A couple of years ago i plaace a halogen heater to the left handside (nearest the cab).  I checked the van before going to bed and all seemed fine,  the outside temp was -4/-5  The following morning i went out to the van & before starting the machine i checked the water in the header tank, it was very cold but not frozen so started the TM.  All was ok however,  my solution hose, garden hose, wand, handtool & sprayers were frozen.  Luckily just 1 of the wand valves required replacment.  So the 2kw halogen heater had kept 1 wand that was to the left of the machine & the TM above freezing.  Everything further to the back of the van was frozen.  The following evening was due to be -4/-5 again so i placed the heater in the middle of the van & the following morning everything was ok although there was ice on 1 of the male quick release connectors that had slight drip on the front of the machine.  The van was a Transit Jumbo so there was alot of space for the heater to cover.  Had the van been medium wb i think it would have been ok.  Just thought id let you guys know of my past experience.  I think that the van really should have had 1 heater either end of the cargo area.  I also think that if i had placed a blow heater into the van instead of the halogen heater then all would have been ok.
Richie.