Maybe your truck has a smnall small short, that has gotten worse to the point where the batt got drained. Or a crappy connection to one of the batteries posts or main cable connections. Or your battery just died.
Does the ignition switch feel fine? Mine once spilled its guts out the back if I recall, and I had nothing. (Besides a new problem!!)
let us know what you find..
"Yeah, make sure its a flatbed...."