Re condensation, couldn't you have a small fan running extracting air? surely these days you can get very quiet fans and mount the fan out of the sleeping area?
Still need fresh air vents in though, however I'd imagine they could be small as you are actually sucking air in
PC case fans would fit the bill for that sort of thing. They run near silent and on 12v.
I've gone a similar route for my insulation.
My last van I went for the best (and arguably easiest) method, and thats the two part spray foam kits. You get your windows in, and any battens stuck down. Then mask everything with cheap gaffer tape/sheet over cab etc and spray the whole lot in one go. Sticks directly to the panels with no air gaps so all worries of vapour barriers etc are null and void, deadens panel noise and won't absorb moisture.
On this van, I have done exactly the same as you except I used free 20mm non-silvered insulation boards, and I bagged my bottle insulation in clear Screwfix sacks, sucked the air out with a hoover and then posted them into the cavities where they expand again. I chose against a full on vapour barrier as 1) there is the real chance of an undetected leak soaking the insulation anyway and 2) varnished plywood/carpeted plywood will do a good job of that. On my old van I never found any evidence of condensation on the parts I didn't spray foam, and those areas (such as around locks) just had bottle insulation stuffed in there.
The only big difference is I took a different approach with the floor. Old van had battens and kingspan, it was warm but robbed about 40mm of headroom, and was never entirely creak free.
This van, I've done the Rolson closed cell foam matts with a floating ply floor over the top. Nice and quiet (in terms of road/drivetrain noise), no creaks, and about 20mm off the headroom. The mats are in Halfords but cheaper on ebay.
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Looking back I think I'd have gone the spray route too - for the time / effort / cost it's a no brainer.
I've finished my insulation now apart from the floor which I'm still going down the 2 sheets of ply with celotex inbetween method. it's all cut to shape I'm just holding off laying it until I've got my drop vents positioned and unwin rails / deadlock in the back
I'll get pictures up soon just been manic with work and vanning!
The solar panels are on too! woop.
Interesting idea with those foam mats, I may fill the lower parts of the floor like you have done still as I've got some of those mats in the garage not really being used.
Edit - what are you doing about your wheel arches?
