I like the look of that wood you're putting up so much that I'm going to do something very similar. That makes it look so much more like a home. You guys work good together.
I haven't actually answered the question of doing just the rear sleeping area or as much of the floor as possible. I'd say that largely depends on how much of the underside I'm able to effectively spray without other things being in the way. For larger flat areas maybe I could affix chicken wire to the bottom of the floor in between the floor joist. It's very rust resistant and would act like tiny re-bar in the foam. It sounds like a PITA to affix something for the foam to cling to, but I think the spray foam may have a tendency to eventually delaminate from the underside of the floor due to vibration and moisture. Even chicken wire stuck up on the bottom of the floor would be better than just depending on the foam sticking to the underside by itself.
So I'm still undecided about doing the whole floor or not. I have pretty well convinced myself that I'll need the 1000 bf package of foam. I like to err on the side of excess, rather than not having quite enough to do the job. Besides, I had cold feet last winter.
I think bed liner would be a good sound reducer on the underside of any bus. It's well known to have sound deadening effects, not to mention the rust prevention. After I've applied my foam and it's cured well I intend to do a test with bed liner on the foam surface with the idea of helping to reduce water absorption. That's if there's no chemical reaction between the solvent in the bed liner and the cured foam. Yeah I know it's purposed as closed cell insulation, but it does seem to sometimes hold water. Maybe that's just old foam. IDK. It's just not possible to take every precaution. If this last for 20 years it will have paid for itself many times.