Cat 216b2 skid steer won't hold battery charge

Hi everyone. I'm new here. My cat 216b2 skid steer battery won't hold its charge. i think its a 2007, only has about 1200 hours on it. I've had the battery and the alternator both replaced and the problem persists. It starts if its boosted, or if I put a batter charger on it for 30 mins or so, runs fine for as long as I need it, it'll start within the first hour or 2 after shutting it off. But if for example I try to start it hours later, or the next day, its completely dead. So dead that the beeping when you turn the key to the on position is very weak. I've also noticed that if i have the lights on, or the heater on while running it, it seems to drain whatever juice was in the battery from being boosted and the machine stops working. Stays running, but loses all controls. If i don't have any electrical components running i.e. heat, headlights etc. it will run for as long as I need it. Is it possible theres a short somewhere in the electrical system? Any tips on finding it? Or how to test for it? Mechanic can't figure it out, and a cat dealer is a couple hour drive away.
Thanks

Cat 216b2 skid steer won't hold battery charge