Electrical problem

[Mav]

Am out of ideas, hope somebody has a good one. My 70 K10 has a new battery but when it sits overnight am down to 11.5 volts and wont turn the starter. I put charger on for 1/2 hour and its 12.5 , starts fine. Thought I had bad battery off the shelf so put in 2nd new one, same problem. Alternator is putting out 13.5 volts, (had shop verify, and they said might be regulator so put new regulator and condenser on, didnt fix. Battery gauge hadnt moved in years, stumbled on info about the 4amp fuses in front corners of engine compartment and replaced them. Only draw on battery is clock in radio and its .2 millivolts. Battery gauge now moves toward discharge when starter is engaged. Have disconnect on neg. batt. post but batt. drops overnight whether its on or off. What havent I done? Thanks, Mav