Do you have a small solar panel. That could fool you.
With things connected push your emergency start button. The one that combines things in case you need to boost your starting batteries.
Listen for the clunk and find the thing that goes clunk. It might be a shiny solenoid about the size of your fist. You can feel it clunk when pushing the button.
This should be done without the engine running but the key may need to be in the run position.
This clunking device could be used to combine the batteries while running down the road. If you find one then carefully tighten the large nuts on it. Disconnecting battery negative cables is the safe way. When you tighten any nuts always make a mental note of how tight or loose they are. If it takes a couple of turns you could easily have found the intermittent connection. In any case, the solenoid is old enough IF you have one that it should be replaced to to the intermittent nature of your problem.
