I would fit a indirect caravan gas heater which exhausts below the van,requiring a hole cut in floor but cheep as chips to buy ,or a diesel night heater,both are safe.
Never a open flame other than for cooking,at night you may fall to sleep,forever.
No loo what the feck do you do through the night cos im up at least 3 time to point percy at the plastic,no sink either where do you wash cutlery or ones self.
You will find it colder with single bonded windows but you can use the foil covers to stop that and condensation,i do agree seitz are way over the top on price.
Best post so far,seen a home made unit in alloy with landrover type 4inch roof widows in each side,looked the bogs dollocks.
A grp unit could be made but time may be a factor,though there may be other things that cam be adopted.
Now where did i put that old tin bath.
3/4 years :lol-053:there just about run in unless you are buying crap,my car cost £10 when 7 years old ,now 22 with over 305000,cant beat a skoda & never missed a beat.
My vans now 16 years old and im 100% sure there is another 20 years in her,iveco.