There was not much that was necessarily wrong with the frame and sublfoor of the trailer, but since we're building this thing for longevity, we decided to take the time to refurbish them anyway.
The first step was to remove the factory OSB subfloor, which was not as easy as one would think. We used a circular saw to cut it into large sections that were easily removed, but that left the edges which were wedged into the aluminum C-channel that makes up the base of the Airstream's shell. Those edges were held in place with various screws and bolts, all rusty, which had to be ground off with an angle grinder, before we could painstakingly pry the remaining OSB pieces out.
While we were at it, we took the opportunity to clean up the surface rust on the otherwise beautifully solid frame (which was the primary reason for purchasing this trailer, considering our old trailer's frame was rusted beyond repair). With some wire brushes, we removed the loose rust, and cleaned every inch with some cleaner/degreaser. We then used some Rust-oleum Rust Reformer (which an informal, non-biased government study seemed to idicate was the most effective product in this category) to neutralize any remaining rust, and then finished it off with a topcoat of some standard Rust-oleum enamel paint.
With the subfloor removed and the frame fully exposed, we also had the opportunity to prepare our fresh water and waste water tanks for our new layout. We repositioned one air vent, closed off another, sealed off the toilet flange, and ran some new plumbing into and out of the fresh water tank. We also added some modern tank level gauge sensors which will eventually be hooked up to our monitoring system.
The new subfloor is built from eight sheets of marine-grade 3/4-inch plywood, which will resist floor rot much more than the OSB. But for added security, we painted all the edges with a polyurethane porch and floor paint to seal them off and prevent any water leaking through the walls from soaking into the wood. Before closing it all up, we filled the gap between the belly pan with fiberglass batt insulation. To finish the job, we used stainless steel bolts with locking nuts to reattach the the frame, C-channel, and subfloor to each other.