Any thoughts on foam board or spray foam?

Spray foam is toxic

Yes. Why would you ever put a toxic, petrochemical-based material in a confined space? Foam is a nasty material that should be discontinued. It does not address moisture challenges, is unsafe to breathe, makes noise when driving and is likely to spend a few thousand years in the ocean once your van is re-purposed. Please use common sense and don’t use foam. Foam board is cheap! we get it. you may find that the lure of cheap materials is outweighed by the headache and time needed to install. Havelock Sheep’s wool is quick to install, you can’t get it wrong and is so much more effective in the real world environment of a van facing moisture challenges. Using 15 cans of “great stuff” to glue the foam board in seems very counter-intuitive to a healthy space.