WBCCI can help you with selecting a number
When I bought my Airstream it had an old number on it that the previous owner did not have for at least 8 years. I contacted the WBCCI membership director through email and found the number was in use. I asked if there was a 4 digit number available and the director suggested the number I now have which is the same as the number that was on my trailer except for the first digit.
2015 Sprinter Class B Camper Van
(Former 1971 vintage Airstream Owner)