We've owned a couple of Jaycos. It says right in the manual to inspect for rust and recommends cleaning it up and painting the frame. Starcraft is owned by Jayco so I imagine the manual has similar wording.
The reality is that your trailer may have indeed sat in an open field. If it was a lot trailer it sat at StarCraft's lot for a time, then your dealer may have an outlot somewhere where trailers are stored until they are preped. Our trailer was built 6 months before we bought it. There is some rust in a few spots and the brackets for the propane tanks are really rusty. Its clean up work.