Electric fuel pump? More lights than the circuit can handle? A high resistance short that isn't bad enough to blow fuses yet? Too big a fuse in a shorted circuit? Air conditiong compressor locking up? Starter solenoid stuck? Shorted battery? The only way you're going to find it is to dig in and start checking.
1993 F-350 crew cab non-turbo 5 speed transmission
35 years a mechanic on large stationary engines natural gas and diesel fired, and other industrial equipment.