I personally don't know what Winnebago offered as optional equipment back in 1992 but all is not lost. Winnebago does make available production information for their products going back to 1990 on their web page.
Winnebago Product InformationI did a very brief look at the electrical diagrams and didn't see anything with regards to an powering an air compressor or other such needed wiring for an air horn but I could have missed it. The brochure for the coach from 1992 didn't list an air horn as optional items. Closests air type option was rear air bags on the Chevy chassis but no mention of a compressor and tanks.
2006 Hurricane 31D built on a 2006 Ford F53