I'm agreeing with Luke (Mechainiac) on this. Since it is a fire truck, it is probably a low mile engine. I would start by assuming stuck rings. Put the plugs back in. Maybe Luke can tell you an easy fix for the one you pried out. Then warm the engine up and go for a drive on a deserted road. Up and down hard on the throttle several times usually seats the rings. It's the action of the vacuum and the pressure that helps move them. See if that helps the problem, it's a quick and cheap fix if it works. If not, you're in for a rebuild.

Fred
52 3600
69 C-10