Keeping the cold air out of the van is essential. It doesn't help that the R20's rear door is notoriously ill fitting and so allows for the wind to whistle in. Floor length rear-door curtains are an absolute must.
Our curtain rail sits just below the rear light and is held on with two, very strong, suction hooks
A simple tie-back which velcros to close and also has a small velcro pad on the outside to stick to corresponding velcro on the van and so holds the curtain and fly-screen mesh back in place when not used
There's also a couple of bits of velcro on the opposite side of van with corresponding velcro on the curtain and fly screen so either or both can be held in place across the door as needed.
A tip - when emptying the loo, tie the curtain back and then lift the bottom half of the curtain up onto the sink. This gives clear, unfettered access to the loo compartment.
So that's the back door draughts taken care of. The other major source of cold is from the cab. The van does come with curtains to the cab but these are both quite lightweight and also not long enough to drape right to the floor and stop low-level draughts. Easy to replace with longer heavier curtains and the 'bobble' caravan tape such as this from Amazon
However for us it made sense to have another floor length curtain hanging from the driver and passenger seat headrests. This doubles the insulation and provides handy storage pockets, including one cab-side which contains our hi-viz vests, thus conforming to laws which say you must have access to such from the driving position.
The curtain is simply suspended on a curtain wire which is attached to the front seat headrests. Well, not quite so simply - it's a heavy, insulated curtain and so weighs down on the wire bowing it to quite a catenary. Some form of stiffening is required. I found a thin fibreglass rod, probably a kite spare or similar and it is almost the perfect length to slip in the curtain channel next to the wire and provide the necessary stiffness. I've just poked an end out below so you can see
The above works for us.
For the moment! As always we are playing, exploring, experimenting on ways to improve comfort in Brian The Tardis Snail.