Ive used my homebuilt camper van just for weekends etc. through the last 3 winters. Being in SW Wales and the South generally and near the coast though, I don't think its been WAY below freezing even on the worst of occasions.
When I first got the van... it was pretty cold inside in winter, and without the Webasto on at night you'd be too cold to sleep even with a thick duvet.
I've never had issues with fresh water, as my water tank is inboard, the loo is a Thetford thingy with a top and bottom self-contained type, and my grey waste requires me to put a catch tank externally - so there is no onboard grey tank to freeze up.
But it was still cold!
I've slowly added things to help.
I replaced the flooring with laminate, and put some extra insulation with reflective foil under it.
I got rid of draughts by using the old sausage style excluders in the front footwells
I added a load of extra insulation (the heavy dense foil-faced stuff they put under laminate floors in houses) under the cab rubber matting, and then put some thick carpet over that to try and reduce cold transfer up through the cab floor
I found that the area under the kitchen units hadn't been insulated at all - so I lined it with more of the heavy stuff above.
I covered the folding bed base panels with more of the above, and then stuck carpet tiles over that - made a big difference in retaining warmth in the mattress and insulating the bed base from cold coming up through the floor (I also cut 2 sections of the heavy ins to act as mats which I can lay under the bed sections as it is drawn out to add another insulation layer - when the bed is away they just fold in half and sit under the couch section with the duvet storage).
I made some internal silver screens to reduce the heat loss through the windscreen and front doors, plus another for the sliding side door window (mine are internal and not as efficient as outside posh ones, but at least I can take them up and down from inside the vehicle - handier esp. when wildcamping) and they help quite a lot. I also have some curtains that cover the windows - mine aren't thick /heavy enough really but it does add an extra bit of insulation. More benefit could be had by adding a heavy full length curtain to pull across and divide the can area from the main hab area at night... but I've not actually done this.
I made more silver screens which attach with velcro to the lining carpet on the roof - mine are only to cover the opening Fiamma roof vents as these don't have anything in the way of insulation so will leach heat badly - covering them with a silver screen helps to keep early morning light OUT, and warmth IN. I have considered making much bigger ones to cover larger areas of the ceiling at night - as this would boost the insulation value in the roof overnight.
I can say that these tweaks have made the van a lot more comfortable and it has been slept in like this in below freezing temps overnight - without the heater on at all. It is a bit chilly in the am or if you pop to the loo in the middle of the night... but not bad at all - and in the am I just pop the Webasto on to take the edge off along with the kettle, and just hop back into bed while the kettle boils and the heater makes the space toasty.
Note that most of these things don't actually involve messing about with the main fabric of the vehicle - so they are easy to implement.
I hope this helps.
G.
