Is there a good mechanic out there that knows whats wrong with my car.?

I have a 1978 holden statesman deville. Its been a great car but lately i have to dismantle battery every time i drive or my battery is dead by the morning. It does not die while driving. After a long drive and i turn the car of it wont start for about 20 to 30 minutes. I have asked many mechanics they don,t seem to know whats wrong want to send me to a electrical person who says its will cost quite abit of money. It is not the battery as ive taken them back 3 times something is sucking the life out of it don,t know what. The mechanics checked starter motor. has anyone else had the same problem. Ive had the car for 20 years.