My take is there WILL be rust behind the interior panel. But probably not enough to worry about. You could get the cab dipped in rust remover/treatment, but that would be $$, and wouldn't help with rust-thru pits (if any).
If you want to peek in there, you could get a borescope and thread it in thru the top. I've got one that works with a laptop or cellphone (has a wifi transmitter) that I got off Amazon for about $25.
