We first see John's Truck at the end of 1.16 Shadow.
It is also seen in Dead Man's Blood and Salvation. The truck was last seen parked on the corner of Wabash and Lake, outside the warehouse in Lincoln, Nebraska where John went to meet Meg in Salvation.
The truck is a 1986 step-side GMC Sierra Grande. It's almost certainly an automatic and most likely a 4x4. (A definite 1986 GMC Sierra Grande for comparison.) It has a well-stocked weapon's compartment.
John's Truck has been nicknamed Truckzilla by fans.