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I would be very hesitant to use foam of any kind in the walls. The walls, in fact the entire coach, "work" a tiny bit as it travels, or just from heating up and cooling off everyday. This slight movement is likely to cause foam to crumble and self-destruct. Fiberglass R-value can be increased slightly - slightly now - by stuffing in oversized batts. That is to say, a standard 3 1/2" batt made for stud walls can be stuffed into the 2" space available. You will not get R-11 out of it, but it will be better than R-6.
There is a problem with fiberglass in the belly areas as it gets wet, stays wet, and loses insulation value. I would think that here the closed-cell foams might be just the thing. If not, I'd be interested to hear why not.
The floor in an integral part of the structure. It is not just something to stand upon. Airstreams are unique as being a monocoque structure, and the floor is the element that ties it together. My point being that you need to use as large of panels as you can to minimize breaks in the continuity. Also, there are panels that are not particularly subject to rot, but tend to come apart if wet for long periods. I'm thinking that MDF is one of those?
Mark
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