That lettering was definitely hand painted, not a decal. It was very common back in the 1950s and 1960s to personalize your Airstream with you names, home town, and possibly some of your travels, especially on Airstream caravans.
Howard and Irma Hertenstien were Wally Byam Caravan Club International (WBBCI, a.k.a. the Airstream Club) members in
1967 who lived in Indianapolis, Indiana. Their big red number that you might find ghost outlines of above the front and rear windows was 10970. They had no recorded caravans as of
1967. I could do some further research to determine when they first joined the club, how long they remained members, and if they went on any recorded caravans, but not tonight.
I agree with an email reply you have received from Scott Goranson that polishing will likely completely remove this lettering that was created by the aluminum/clear coat under the painted letters and symbols aging/etching less than the unpainted areas. You could chose to have those areas repainted to preserve them. If you want to polish the trailer and then repaint the lettering there are ways to do this, but it is too complicated to detail in this brief reply.