I did my last van with the foam but over did all the insulation with 10mm and 6mm foam, tin foil bubblewrap and plastic bottle from B&Q. It still got chilly inside due to the 5 windows. This time I'm doing 3mm foam on the roof and side panels, this will make it quieter and stop the condensation and as I shall be moving to Portugal later in the year I don't expect much damp in the van.
I buy the sticky one side for ease of fitting and expect the 3mm to stretch a bit. It's just wash all the paintwork so it sticks well.
I have the condensation issue but also had a leak before that, is it worse on cold days or really wet days? Cold days I have condensation but one summer after a really heaver downpour I seen that I had soaking wet ply and water dripping down my seat belt, turned out it was the roof bracket nearest the front drivers side. Someone before had put too much weight on the roof rack or used a ratchet strap too tight and it had actually lifted the mounting bracket off the roof ever so slightly. It required a little weld and a lot of silicone and it seems to have stopped the leak, that was about 5 years ago and not had an issue on it since. Worth having a look round the mounts to see if there's any issues there. Hope this helps.