Twin FWC 7.4L Mercruisers w/Borg Warner straight drives
Turn it on and get a multimeter and check the battery voltage at the battery terminals. Should be around 13V-14.5V with no load on the batterys. 12.5V or less if its not working.
The second part to this is whether or not it's cycling off at the proper time - otherwise it can cook the battery. But honestly, at over 3 decades old, it's way past it's due date. Replace it now before it does kill the batteries. Chargers aren't very expensive and the newer technology is light years ahead of what was available in the 80's. Chalk it up to preventative maintenance.