To explain the phenomenon - batteries will, to an extent, recharge themselves if left alone. They will NOT generate a full charge. So you're getting enough power to light the lamps but it takes a lot of amps to spin over any engine and especially a Diesel.

Sounds like the battery is OK or it probably wouldn't regenerate itself. I'd check the alternator for sure. Pull it off and take it to your local alternator shop. They'll spin it up on a test rig and evaluate. They can either rebuild yours or they may have a rebuilt sitting on the shelf. My local shop ALWAYS does a great job for me and charged me $100, which is a pretty good deal. They stand behind their work 100%.

Also take a REALLY good look at your cables and the state of your grounds. Remember that EVERYTHING (the chassis, body, and engine) have to get a complete ground loop back to the battery. Many electrical Gremlins can ultimately be traced back to grounding issues.

Dan