I did a lot of calculations on thermal efficiency for my camper, but I can't find them. In building a camper there are two problems, one is keeping heat in, one in keeping heat out. Then there is thermal mass - as regardless of how efficient your insulation, during the day , it will heat up and you don't want to have hot walls in the the camper overnight if you want to sleep comfortably.
My camper has an aluminium skin over steel. The aluminium is stuck with a 3mm Sikaflex 252 layer. The aluminium is insulated with a 10mm foil/bubble wrap insulation, then 50mm of a Higgins insulation rockwool/composite that was developed for the Queensland Bus fleet. It has sound and heat insulation properties. It also meets all the safety requirements, no toxic smoke, self extinguishing etc. Do a simple test for any material you are considering, set it on fire with a blowtorch and breathe in the smoke, if you feel sick and it doesn't go out by itself, walk away from it - do a search for Grenfell and see what effect choosing the wrong insulation can have. I would stay clear of any Polystyrene in any form, it emits poisonous fumes and once alight, does not go out.
My entire camper is lined with a composite of 1.8mm marine ply, 6mm aerogel foam and 0.8mm white Laminex. it is stuck to the steel frame with minimum of 3mm Sikaflex. I set a bit on fire with a blowtorch, the Laminex would not catch fire, the plywood did after a while an it went out when I stopped applying the flame, which is good, and the smoke was not bad to breathe in , although you would not like doing it for long, but didn't make me feel sick, crude test I know but works for me.
The floor is also a composite, with an outer 2mm aluminium skin, 10mm of aerogel foam and then 6mm of marine ply. Then the is another 4mm Jarrah wood veneer on the visible parts, part under the drawers have a 5mm marine play +0.8mm Laminex outer layer. This insulates the floor as well.
I have only the Dometic double glazed windows, I would rather have something more substantial, but they were not available in the sizes I wanted. Most European design campers have very little in the way of windows, and I find them quite claustrophobic, especially the ones with high cupboards around the roof.
I also have a Websto Thermo 90ST 9kW diesel heater and two Kalori 5kW heat exchangers. This is more than adequate for heating our camper down to -20C if I remember my calculations. Here, we seldom have it on for more than 30 min before it gets over 25C in the camper. I have a thermostat for the fans on the heat exchangers which shut off and then just convection keep the camper warm. The windows on the cab of the truck also gets insulated, to minimse heat loss there as well.
Hope this helps Simon.
