Two things, weak batteries and bad isolation/charge solenoid.
If the batteries are old or low on water, they won't last a day. But, while driving, they should be charging.
Have someone push the boost or Aux start button/switch on and off, while you listen for it clunking.
Once you find it, replace it. They fail ALL of the time.
Be sure to remove the negetive battery cables of all batteries before working on this solenoid.
Its a contionues duty solenoid, be sure the replacement is too.
May look like this.