Truck won't start

We have a 2008 Chey Silverado 1500 Ext Cab  5.3
4x4..... and if it sits for a day  or more without being
turned on it will be dead,.... we have change the
battery twice, the alternator, the starter .....had my
father  Who used to be a mechanic back in the day
pin it out and he could find nothing so  we took it to
the  repair shop  and changed the  stop light switch
(which is what our mechanic said it was when he
put it on the machine)..... and after two months ,he
didn't drive it for two days and it still did the same
thing would not start ......
Please can someone help!!!!!