You will die. There are no resources or ammo in this truck, only death.
Now I probably should have noticed that the truck was shaking. In my defense, I was running away from a completely unrelated (but still kind of related) bad decision and everything around me seemed to be shaking as my stamina bar wore down. Evil Within 2 takes every step to ensure getting out of one jam has a high likelihood of luring you right into another; this is a prime example of that.
My hope was this truck would be a temporary safe space from the horde of demon creatures I just recently pissed off. Turns out it was not. Since being alive is better than dying, please, do not open the back door of this truck. You will die.