The LCD may fog up but will clear up. I've had more trouble with them being in direct sunlight and causing the LCD to "black/fuzz out" more than just heat or cold.
I left my meter laying out in direct noon sun one day in the summer. When I went to use it the display was gone. I just thought the battery died so I replaced it. Still blank/dark screen. I laid it in the shade under some bushes for a while and the screen cleared right up.
So, the LCD may mess up but I don't think the "workings of the meter will be damaged any. I leave mine in the van year round as well.....hot-cold-hot-cold....