We don't have a 'winter' pack on ours but have used it in sub-zero temperatures and the only thing that was a bit of an issue was the slight draught that came through the rear doors where they join if the wind is in a certain direction, soon solved that with a length of foam pipe insulation which fits perfectly in the gap and stays in place on it's own. I also made some insulated pads for the rear door windows (out of an old internal windscreen insulated screen) because the existing concertina blinds are just too easy to damage on a night time if they get hit with the bedding and make a hell of a noise if you do! The pads are easy to put on/take off and obviously add some insulation to the windows, I've also made a pad for the window at the end of the bed too which can be used it looks like it's gonna get really cold overnight. I already had pads for both roof vents which I used in our previous PVC as well which give insulation, from both the cold and the heat too, and on a night time they block out the light much better than the blinds so I don't end up getting woken by light creeping round the sides.
If we are going to go away in winter I quickly put in some thermal curtains on a push/twist fit shower rod that fits nicely just inside the rear doors but obviously in warmer weather I just take them out - as we often have to open the rear doors for stuff having a more permanent rear 'pad' arrangement just wouldn't work for us.
Mine has rear curtains as standard.
Ive had some rear window covers/blind protectors laser cut from perspex
Were away in our van tonight so will see if the extra insulation I added makes a difference.
I may look at making a rear door thermal winter cover if I feel any cold tonight