I think you have an electrical problem. It could be a bad battery that isn't holding a charge, a bad alternator that isn't charging the battery enough, bad cables that aren't getting the juice to the starter, or something in the system that is draining the battery as the vehicle sits.
Does your truck have gauges or warning lights? If the latter has the battery warning light been on?
Do you have a digital volt meter? If so, check the battery's voltage. A fully charged battery should be around 12.6 - 12.8 volts.
If the battery is charged then you probably have a cable problem. Check the connections of your cables coming from the battery for loose or corroded connections.
Gary, AKA "Gary fellow":
Profile
Dad's: '81 F150 Ranger XLT 4x4: Down for restomod: Full-roller "stroked 351M" w/Trick Flow heads & intake, EEC-V SEFI/E4OD/3.50 gears w/Kevlar clutches
Blue: 2015 F150 Platinum 4x4 SuperCrew wearing Blue Jeans & sporting a 3.5L EB & Max Tow
Big Blue: 1985 F250HD 4x4: 460/ZF5/3.55's, D60 w/Ox locker & 10.25 Sterling/Trutrac, Blue Top & Borgeson, & EEC-V MAF/SEFI