Not disputing what Andy is saying... found this...
From VintageAirstream.com:
Frequently Asked Questions
Q: WHAT SIZE WHEELS DO I USE TO REPLACE THE SPLIT-RIMS?
A: The standard for all single axle Airstreams, except the Bambi, is a 15"x6", zero offset with a 6 lug pattern on 5.5" spacing, rated at 2600#. A properly rated Chevy truck wheel will meet this sizing, too. Don't use underrated car wheels. The desired color for originality is a black wheel.
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The brochure for the 18' Traveler says 2100 lbs. overall weight.