I don't plan on living anywhere cold but if I did.
I didn't use furring strips for my flooring. I framed all of my walls screwed to the metal floor then I used the 1/4" underlayment made for floating floors . Each space was floored separately.
My main walk and entrance area the floating floor is held down by the 1/4" round trim on the edges loosely because it is a floating floor. With the floating floor idea don't forget to leave a gap on the edges for the expansion and contraction of the wood with the temperature changes and the twisting of the bus going down the road.
I went underside and cleaned the metal floor bottom around any screw penetrations as I finished a run and caulked them.
If I insulate my flooring whether it be spray foam or 3" thick board? It will be done from underside to cover the ribs and all, save some headroom inside and to me most thermal breaks are more efficient before the metal skin? Not after it. Won't look to good on the sides and such but no reason not to. I am cleaning,priming and building my storage racks and such as I go but with 1day a week at home it is slow going.
Just my thoughts. Ya'll are the only ones I listen to if/when there is a complaint?