Whatever type of van you have it is worth investing in a set of thermal windscreen covers.
While the rest of the windows are likely to be double glazed plastic and reasonably good at preventing heat leaking out, automotive legislation means the windscreen needs to be laminated glass, and this unfortunately means poor thermal insulation.
A set of externally fitted insulated covers for the windscreen makes an enormous difference.
There are covers for some vans available that cover the entire engine bay, these can be useful if there are drafts between the engine bay and the cab, but the windscreen is the main area to worry about.
We use a set from ‘TaylorMade’ in the UK that covers the windscreen and both side windows, fit and finish is excellent.
The front section is attached to the sides via heavy duty velcro, allowing it to be folded down during the day if you wish, allowing light and warmth from the sun in without having to remove the whole cover.
Insulating the glass from the outside also helps prevent the condensation that can build up on the windscreen in the winter.
If you are planning a trip to the mountains (which you should!), or indeed travelling anywhere cold weather can be expected you will thank yourself for investing in a set of these!