Sorry it took me so long to reply! I've been extremely busy with work and what not. I haven't had time to work on my bus in weeks. I did use it to pull a stump though!
Anyways, on my sub flooring, I used 1x2 firring strips, which are actually 3/4" thick. I laid them around the edges of the foam and one strip right down the middle of the bus. After doing all of that, I would think you could totally go without the firring strips, but it's still not a bad idea. I didn't lay down any felt paper or anything either. I DID seal up all of the holes in the metal where the seat bolts were. I used liquid nails. I cut the foam with a big box cutter knife. It wasn't perfect, but nobody will ever know once you lay the sub flooring down!
Also, I used TEK screws for EVERYTHING! I used TEK's to screw the firring strips to the floor. I would pre-drill holes in the wood first so that the screw would just fit through the hole in the wood. I found that if I tried to use the TEK screw to drill through the wood AND the metal, it would dull quickly and not get through the metal. These things are meant for drilling ONE hole. It wasn't too big of a deal. I used two drills. One to drill the holes in the wood, and one with a bit in it to drill the screw into the metal. After I had the firring strips down, I'd put big globs of liquid nails over the seat bolt holes, and then lay the sub flooring sheets on top of that. To attach the sub flooring sheets, I'd again use my drill to drill holes in the wood all the way to the metal. I'd go back and use the other drill to attach the sub flooring to the metal floor. For the sub flooring sheets, I used counter sunk TEK screws. They look like dry wall screws, but with the drill bit ends. It worked out great! I used quite a few screws too. I'd go around the edges of the sub flooring sheets, and right through the middle too. About one screw for every 10" or so. That is probably overkill, but I have a VERY solid floor. I found that around here, Lowe's has the best selection and prices on TEK screws. You could do the entire floor with the counter sunk screws. I was careful to not get screws that were a whole lot longer than I needed. Maybe 1/2" sticks out on the bottom in most places. I spent a LOT of money on screws trying to find what worked the best. Just get the black, counter sunk screws. I think I paid around $8 for a box of 500 screws. Not bad.