The humvee is a truck that is found in Dead Rising. There are two humvees in the game, they are very fast and durable trucks.
The Convicts
One is driven and operated by the convicts, and found in Leisure Park. This can be obtained anytime during the game starting at 6pm on the first day. There are three convicts in the first humvee, Sam Franklin drives, Reginald Jenkins fires the heavy machine gun mounted on the rear of the vehicle, and Miguel Sanchez is armed with a baseball bat.
Escaping Willamette
The second humvee is driven by Isabela Keyes and Frank West during their escape from Willamette, which was nearly destroyed by Brock Mason in Overtime Mode. It is unknown who was firing the heavy machine gun, but it is thought to be Isabela, as she was in the Gunner Seat in the before and after cutscenes, and occasionally screams when hit during the actual battle, but Frank doesn't react at all.