Spray foam really requires you take everything down to the outer skin & ribs to be effective. Because the nozzles will seal within 15 seconds of releasing the trigger, you will need to have as continuous a spray as possible. If you're living in the bus while converting it, don't use spray foam.
On the other hand... Slow rise foam may be a better option. You must be very careful not to blow out your walls, and like the spray foam, you will have to have a lot of spare nozzles available to switch out between sprays. But it's super effective at filling voids.
For both products, take a look at both Handi-foam and Tiger-foam.
If you are going to do any construction on the bus, I'd follow Smitty's advice and just drive to the hardware store first thing in the morning, park it, and just spend the day in the parking lot building. If you can stay at the far reaches of the lot and be circumspect (i.e., not leave scraps lying on the ground while working), no one should give you any trouble. At the end of the day, pack everything back up and drive home to the campground!
Good luck guys!