In this encounter, if you search the trailer first, you will miss the goods in the cab.  You wil have to break the window to enter; searching the cab results in .357 ammo, a magnum pistol, and a headlamp.  Searching the sleeper will net you some flares.

Getting into the trailer requires a crowbar, claw hammer, or a set of boltcutters.  Before you search the trailer, make sure you have a light source equipped; you won't get the option to use one later and the wording of the narrative isn't terribly clear.  If you do, you will find around 90 days worth of food.  If you do not have a light source equipped, you will miss this.

A survivor will approach you soon after.  You can drop your weapons and talk to him; if you found the food, you can tell him the truth (and lose the food) or lie and keep it.  If you didn't find food, he will still let you go.  You can fight him; he fights with a pump shotgun.  Searching his body gives you a machete and a pump shotgun .