intermittent electrical issue killing batteries.

[aggiegrad]

Looking for ideas on where to look for the issue. I have a 68 GMC that I still drive almost every weekend for the past year. It was a daily driver for about 3 years before that until my son left for college. Recently it has started to kill batteries. It never acts like it has low battery until one day I go to drive it and there is absolutely nothing. It checks at 4 volts and wont recharge. This was an AGM type battery and was replaced with another AGM battery. I checked for draws when I put it back in and had no draw and everything seemed normal. I continued to drive it on weekends without issue until one day when it did the same thing again (4 volts and want recharge). Thought maybe it was something with the AGM battery because I have run the batteries down before and never had one that couldn't recharge so I switched it for a regular lead acid battery and everything checked out and drove it on weekends without issue until it did the exact same thing this weekend. Now I know it is something in my truck that intermittently is killing the battery but not sure where to look. I haven't replaced this battery yet as I dont want to kill another one but I cant think what is intermittently doing this while it is parked. Has anyone had a similar issue or knows any way to find an issue that is normally fine.

For information it is still fairly stock and still has the external regulator. I have not made any changes to it for a few years. I did add headlight relays, a retrosound radio and switched it from lights to gauges and switched from amp to a volt gauge about 5 years back but none of that ever did anything like this.

Again looking for ideas before I lose another battery in 4 to 8 weeks.

Bryan