I've made a few adjustments to the set up in the last couple of days, largely due to some of the great advice and tips from you guys, so thanks.
I got hold of some carpet to cover the inside of the windows for added insulation and privacy. It was a nightmare trying to fit it on the windows as I couldn't get anything to stick to the plastic surrounds, thanks to second hand car salesmen's tendency to spray everything with plastic shine, which is basically oil and impossible to get off.
After a few attempts I found the best way to do it was to use thin strips of sticky back velcro around the edges of the windows, then cut the carpet to size and put the furry side against the window so it sticks onto the velcro.
It seems to of worked really well, completely blacking out the windows from the outside, and should add some insulation and cut down on frost build-up. Plus from the outside it just looks as the van has blacked out windows and so shouldn't raise any added attention or give away the fact that someone has converted the van to be lived in.
I can remove all the covers when I'm driving and quickly put them up when it comes to sleepy time.
I've used an emergency blanket as extra insulation for the roof, again I used velcro to keep it in place. Hopefully it will help keep a lot of heat inside and not just act as a huge area for condensation build-up.
Plus it gives the van that whole 70's disco vibe and cheers it up a bit, it was starting to look a bit like a cave with all the grey and black.
I'm still playing around with trying to utilize some of the space on the walls and windows, it pretty difficult trying to do so without leaving any permanent holes or damage to the van itself. At the moment everything can be stripped out very easily, just a case of peeling off the velcro strips, and the van is back to it's original condition.
I've got a couple of trunk organizer type..........things, to hold stuff that would otherwise litter the floor, but trying to hang them from existing parts of the van is a pain. I'l probably leave these down for traveling and put them up once I'm stationary.
I've completely given up on the air mattress idea, and gone with 3, 2 inch thick rolls of foam for a mattress. Although I am still using the un-inflated air mattress as a protective and waterproof cover for it.
I'm probably going to do a trial run in it tomorrow night to see if I have enough insulation and find any other problem areas. I'll let you guys know how that goes.