Hello dazmond
I bought one of these from corwoods, never got round to putting it in the van, and with the pressure washer and gas bottle next to each other, I decided not to bother and fitted it into a friends van who does a few jobs with me, hot water is fantastic but I did say about what your concern is.
IMO.
I think it will crack the glass in temperatures as low as 0 if you have it on that hot, is there a temp dial that you can lower it, ( mine never ) maybe at at lower temp may be ok but I would be bit concerned about it