Memphis: I agree with what Flyboy says. My plywood(2nd choice osb) will be 1/2, or 9/16 (whatever is cheap and close to that thickness at home depot/lowes). For me height is extremely important, so I would love my insulation, plywood, and vinyl flooring to be under 1 1/4" Polyiso is usually R-max brand and looks like this
. IF you use XPS instead, it's not that big of a deal, but I think polyiso has better insulating values. I'm not sure what exactly you're asking about the furring strips, and I'm probably not qualified to answer it anyway since I have no experience with that yet. I'll probably space mine whatever distance feels right. I'm going to do my best to avoid using them for the ceiling but I don't know how well that's going to work out.
HJS26: I'll hopefully have an update for the flooring within a week, but I've been caught up on a few other side projects. I plan to copy how JakeC's Brutus did his subflooring. You can find that here.
The only difference is I'm not going to insulate underneath my shower.
As far as headroom goes, I too wish I raised my roof, but I prefer quitting my job, moving in my bus sooner and letting my back take one for the team over working another 6 months. Next bus(in a dozen years or so) will have a raised roof, this bus is more of a warm up. I think the only thing you can really do to utilize space is efficiency-meaning consider spray foam which has higher insulating values. But I only say this because I'm going to probe foam contractors rates today. I don't actually know if I'll go that route, but if I can foam my ceiling under $400 I may bite the bullet. Edit: I've called about 5 contractors and this may not be realistic. Probably going to stick with foam board insulation.