I've got a 2007 5.4 3v V8 (the worst engine ever made) but if the battery is connected to the Vehicle it will drain the battery me and my dad have been trying to figure it out we still don't know why its doing that it'll start if the battery is charged that and we need to replace brakes,rotors,quite possibly callipers,upper control arm,ball joints, ball bearings and CV axles (there might be more to do but that's what I remember)

 

you likely have a large draw somewhere. Look how to do a parasitic draw test on a NEWER vehicle (this is important as many newer cars have modules that take a bit to fall asleep, or need to see a specific configuration to go to sleep)

Start there..

 

1) have battery (might as well have alt too) prof tested

2). with good/charged battery, disconnect stereo harness and alternator harness. check for draw and/or leave overnight and check battery in morning

report back with results.

edit/add: post up details of any aftermarket elec items (stereos/lights/alarms etc)

 

there's a brand new alt in it and we've bought several new battery's and it does the same thing and my dad (a not professional professional being he was a diesel mechanic when the mechanical 12 valve Cummins was the best Cummins)

 

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Look up negative draw test
That is how you determine how many amps are getting drawn off
Then start pulling fuses one at a time and watching for the draw to go away
If you are lucky, one of the fuses, when pulled will stop the draw
keep in mind that having the door open when you do this testing will increase the draw because the door is open, and the interior lamps will be on
There is a lot to know about draw testing, That is a start

 

OP; this is good advice, you’ll also need to make sure you get truck to go into sleep mode before accurate test.

In my experience, on an unmodified truck, two of the most common draw problems are the stereo and/or the alternator. good luck.

report back with findings/solution

 

You've not owned the worst engine ever made unless you owned a Northstar.