Get the van nice and warm, heaters on full for an hour or 2 or open the back doors and fire the blow heater at it for a bit, then use a sharp scraper and some decent solvents. To get rid of the outline you may need to give the van a mop\cut and polish etc...
I did the last van in the middle of winter on the drive and din;t have access to the usual methods as above, I used a scraper, hairdrier and some spray solvent in a bottle from Screwfix. Did reasonably well, van could have done with a mopping after though.