Well, here’s my take:
I tried to ID it in my usual way(s), but came up empty. I thought it might be one of the Letterhead “Fire Truck” typefaces, but nothing there either.
I used to have a good collection of old typeface booklets, but I sold them all about a year ago. This seems vaguely familiar, so it could be one of those (scanned & vectorized), but it could also be hand-work from the particular sign shop that lettered this truck. Is it possible for you to inquire on that?