Did u check the alternator?
Battery is only storage. Alternator is the one that keeps battery charged. Take it to ur local autoparts and they will check ur charging system for free. Either u have a bad alternator or bad or dirty battery cables
If the alternator checks good with a multi-meter, suspect a parasitie electrical draw or a problem with the fuel pump.
Looking for a Used F-150 in your area?
CarGurus has 223,752 nationwide F-150 listings starting at $1,200.
Search Ford F-150 Questions
Ford F-150 Experts