Generally speaking, you are correct. You don't want the trailer door opening into traffic! Curb side is the side that sits next to the curb when you're parked, and roadside is the side into traffic when you're parked (that presumes "legally parked, right wheels less than 18" from the curb, blah blah blah)...
There has been at least one Airstream custom built, however, with a door located on the curbside front (the standard location) and one door located on the roadside rear, similar to where the rear door is located on my tri-axle, but on the other side. It was apparently built for some specific office-type application. I saw photos of it when it was up for sale a couple of years ago. I should have kept copies of the photos, but didn't think about it at the time.
AIR 2053 Current: 2006 Born Free 32 RQ Kodiak Chassis, & 1995 Coachmen B-van
Former Airstreams: 1953 Flying Cloud, 1957 Overlander, 1961 Bambi, 1970 Safari Special, 1978 Argosy Minuet, 1985 325 Moho, 1994 Limited 34' Two-door, 1994 B190 "B-Van"