if you're garage is attached to house, you'll probably be good with any 1500wt ceramic space heater
if you need an extension cord, be sure it's a 12gage wire cord and no longer than you need
go get some cheap thermometers too and set at various places on the "floor" of the van.
Then you'll know you're completely protected in all areas of the van
back up systems are wise too in case of power failures.
If no non electric back up syetem, in a pinch, you start the TM and let in run for 30 minutes or so, then shut it off.
The heat form a warned up TM will heat a van for some hours.
The cheap thermometers will tell you how long
It might save your bacon in a blizzard or ice storm power outage
..l.T.A.