Its a little more difficult to build a lot of sound pressure on a vehicle with no roof. And frankly, we never measure the SPL of our vehicles, since that is not the customer we cater to. We mostly deal in high end gear in high end vehicles, not walls of woofers. (Nothing wrong with walls of woofers, just not our niche)
This vehicle wasn't designed for SPL, or even for SQ. This was simply designed to be loud, clear and heard from far away to bring people over. If it makes you feel better we do tune most of our vehicles with an RTA.
You have to imagine the truck in its natural element, where it would be cruising up and down by a beach somewhere, slowly, with the music cranked up and people partying inside. Like a rap video.