POR-15 likes the texture of rusted metal, to get it to bond well to new metal it should be degreased and etched (of course they sell products to do just that) so that could be a point of negotiation w/ trailer shop to deliver trailer either unpainted or you provide the paint, etc. POR-15 is fun to use but not that fun.
I agree the $2800 may be just a starting point; wait till they seemingly can't reuse the wheelwells or really miff one or more dimensions essential to having the shell bolt on easily so they re-invent the wheel and build trouble in. How much weight is in their replacement? If your old frame, axle and floor weigh 1500
pounds (fictional number) by what percentage will they 'over-build' the new frame? Andy has said there is no limit to the number of out-rigger supports one can put on an Airstream, I would take the time to make sure they understand the out-rigger : bannana wrap : shell attachment model since as you've dropped the belly skins it may not be obvious..
The one-inch spacer is simply how they recommend installing it, that is, how it was tested to get advertised high 'R' value; note that applies only in the horizontal plane and perfectly sealed with the metal-foil duct tape so when applied on walls or curved surfaces the efficiency drops. Also the bubble type insulation is fairly transparent to sound - I chose the Prodex foam type (http://tinyurl.com/ofwg4
)to help restore the noise-proofing the old fiberglass batts provided in the trailer.
You won't put a 1" air gap between the floor and frame; the original compressed fiberglass ended up about 3/16" thick so thats a good measure to try and duplicate after the floor bolts get tourqed down. I arbitraily made the 1/4" thickness for air-gap space but doubled that in the centers of floor area as seen from beneath so any liquid water will drain to the center and out through the staple holes. If & when the shell comes off everything I've done most likely will get discarded but a 1/4" frame-floor gap will line up with what has already been done.
Take the time to write out design points and questions - I am nearly overwhelmed with details that I didn't document. The discipline required to keep a daily diary and brainstorm flow-chart is nothing compared to the let down of missed opportunities and re-working dead ended ideas in the future... I certainly welcome reading and contributing to your posts when I feel competent to do so