Stainless steel brake lines have been available on SOME high end vehicles, and as after market products for decades.
Anyone that wants to install them on any of their vehicles can do so as they can either be bought or fabricated by any one that is handy and wants to spend less than $50 for the tools. The tubing and fittings can be bought at an auto supply store and the tools needed are a DOUBLE flaring tool, a tubing bender, and a tubing cutter.
Stainless steel brake lines do cost a FEW dollars more than the standard galvanized steel brake lines but on a typical RV I would guess that the increased cost is only about $40 for stainless over galvanized.