You are right, the switch went away with the change to modern converters. Sadly, Airstream did not take the trouble to update the manuals.
The simple fact is that the wiring diagrams in the owner's manual are only an approximation of the actual trailer wiring. I have had 3 Airstreams and in each of them the wiring did not match the diagram. As an engineer, I can certainly those figure things out, but as an purchaser, I also would like the information that I am given to be correct.
John W. Irwin
2005 Classic 28 "Sabre-Dog III"
2013 Silverado 2500HD Duramax/Allison LTZ
WBCCI #9632, TAC TX-10