For starters do you have any accessories installed and is this the original battery or do it with new ones as well?
What is charging voltage idle, or with lights on?
Could be a number of things,.. even a bad winch relay also.
With the key off,.. unhook your ground wire of your battery and put one end of a test light on the neg post and touch the other end to the cable,.. does it light up?
With the key shut off the test light doesn't light up. With the key on and the clamp grounded, the test light lights up when touched to the neg terminal. Is this normal?