Yeah! I did use spray foam insulation for the ceiling and walls. I think it was a set from Amazon that costs me about $700. It did pretty much the whole bus, but I started front to back and it really started to run out towards the end. I have the link here, but it doesn't look like it's available on Amazon anymore, but that should give you an idea of what to look for if that's the route you want to take.
For the floor, after I sealed it I laid out a grid using 1×2 furring strips and put 1" foam board insulation between. Then put the plywood on top. I'll be putting cedar panels once I have everything else built.