An old thread, but still, ...in houses it is accepted as BEST practice to insulate OUTSIDE the existing brick walls, rather than inside
If you insulate inside you will inevitably get cold spots, and that's where the moisture will condense, causing damp, mould, etc. etc. all in hidden little corners where it's difficult to see and difficult to deal with.
If you put your insulation on the outside then then condensation isn't a problem because you raise the temperature of the inside of the walls above the dew point.
In a vehicle, the same applies: insultate inside and you have to be VERY careful to stop ANY air contacting the metalwork, especially in hidden places like under the flooring, because you'll get massive amounts of condensation which will form liquid water in places where it is very difficult to dry out. Closed cell spray foam allows provides insulation with total exclusion of air from the metal shell. It's not cheap, it's potentially messy, and it's not a DIY job, but it works.
Now in theory, you could use closed cell spray insulation on the outside of a van, and that would work very well, but it would look very strange. Under the floor and on the roof it could be applied without necessarily looking too bad.
The space under a vehicle is often in fact a rather calm and peaceful place. Usually quite dry and always with a very stable temperature. You can keep your beer under there on a hot day and it will stay pretty cool.
The problem under the van is that there are all kinds of pipes and components and so on which you need to keep clearances around. It's still worth insulating the areas you CAN insulate though - it never hurts. Closed cell foam wil protect metalwork just like vehicle underseal I reckon, and you've already got the underseal on there so I really don't anticipate any issues with moisture or corrosion. And if you later find you need more space to service components, the foam can always be cut away. I wouldn't enclose it in metal sheeting or anything like that; I would treat it as thick foam underseal.
But to save headroom, why not use closed cell spray foam on top of the roof? That will GREATLY reduce the risk of water droplets falling onto you on a cold night from the ceiling, and will make a massive difference on a hot sunny day.
Applied by professionals it usually ends up as a fairly even and smooth layer. It's really quite tough, and you can paint it body-coloured to protect it from the elements. UV will be more of a problem than rainwater I reckon, but paint will deal with that issue. Just make sure that water can always run off OK.
The problem with insulation foam becoming waterlogged (in boats and so on) generally only occurs in confined spaces where a cold surface promotes condensation from moist air and the foam then soaks up more and more of that water over time in a plce where there is insufficient ventilation for it to evaporate off again. A van rooftop is very unlikely to ever trap moisture like that, ...unless you want to go down the turf roof route, but I don't recommmend that because of the weight of the soil.