The batteries you use are they Heavy Duty? is it 4>>6 Cylinder? did you service the Motor? From what you state I would have said Batterys, are the batterys the fill up type? are you giving the motor enough of a run to re-fill the batterys with charge? Fan Belt! is it tight? Alternator, is there a faint Ign Light on the dash indicating a problem, causing the battery not not to charge, You say "YOU" changed the batterys, did you make sure you cleaned the terminal, tighten the connecting wires, and made sure the Earth wires have a good, clean, body to earth from?