If you want a really serious blaster, order one of these kits:
Deluxe mini train horn
Black train horn (Large one)
Both have compressors and a tank.
Train horns and trumpets
I've ordered from this place a few times. (The motorcoach is pretty much a Kenworth chassis)
I have two foot long Hadleys on top of the coach, and those will peel the paint off anything in front of me. (Although, it has a full air system to run them with)
I have a Wolo Bad Boy behind the grill of the Jeep with the outlets aimed out. It ain't bad, and is better than the OEM horn, but someday, I'm gonna' order one of the train horn kits.