Try sitting in the back of a windowless van for half an hour if you are undecided on this
Our van was Coach-built so all the windows have purpose made double blinds with fly screens which we tend to leave down for privacy.
As far as heat loss through the window area goes well yes there must be some but to be honest it is negligible in relation to the overall heat loss from everywhere else. The insulation to the walls and roof and floor is scarcely 2" thick ! so the heat loss everywhere is huge. You need to keep these things in perspective.
Besides we have gas central heating and a gas water heater in ours and we usually take a small electric convector heater to leave on all night if its chilly.
I understand your need to have a flexible space especially for carting stuff around but to be honest I think its a mistake. The compromises you will need to make and the faffing around stripping it all out and putting it all back in and joining your waste pipes water feeds gas pipes etc etc its just not worth it.
Why not consider fitting a tow hitch and using a trailer to haul your stuff. you could even borrow a horse box to move larger items.
