I've read articles about that, he said they were converted to propane to use with the sirens.Don Garlits has one of them in his museum in Ocala. I saw it when I took the tour about 10 years ago.
Diesel needs to be treated for green slime if stored more than six months or so.They used a "chopper wheel" to generate the sirens frequency, not a compressor into a horn. They tour on a trailer to shows. I can see propane being used as sitting fuel for a month is bad in anything. We start our diesel generators every month at our central offices for phone backup. IIRC diesel is very stable over time if kept airtight.