Think it might produce too much fumes and cause condensation, apart from the fact it's a gas burning flame in the back of your van.
I'm sure you should not be using it in a confined space as propane needs plenty of oxygen and enclosed in a van it wont get it so carbon monoxide will build up 
Do a quick google check and see the problems.
exactly my thoughts
it will get VERY HOT in the van, they chuck out a lot of heat, i have used them on building sites and they will heat a lounge size room nice enough, i used them in a portacabin ( which was used as my office and the lads tea room ) 1 of them would heat it in the winter so that i didnt need a coat on
my big concern would be the flame in the back of the van when left unattended
the build up of fumes would make driving the van first thing in the morning a laugh to say the least
saying all the above, the idea might be to just have it on for 20 mins before work to thaw anything thats frozen, if thats the case, it'll work great for that