Though still in process of my own build, I'd started laying some flooring/insulation using screws and liquid nails - but I'll be taking that back up. (Fortunately there wasn't much put down)
The way I'll be doing it now will have a small gap between the panels and bus body (probably 1/8" to 1/4"). Where one panel meets another, there will be a 1/8" gap but also a notch cut out of each panel and an insert cut to fill those notches between the two. This should keep the panels from shifting too much.
To help minimize side to side shifting, I'm moving away from liquid nails (which hardens over time and will break free from the metal) and instead will randomly place some glops (that's the technical term, lol) of clear silicone. It'll stick to both the steel and wood, will remain pliable and help prevent noise. I'll also be putting random lines of silicone between the panels and bus body, filling the gaps in those areas. I'll probably put a 6" line every couple of feet.
Hopefully that all made sense as it's late and I'm whooped.