Look for relays for extra things. I had hella driving lights on my f-150 and the relay was bad. I went through 2 batteries. I tried forever to find it and then I was pulling wires and the relay was going on and off, it was on 24 hours a day. Needless to say I no longer have lights, they didnt work too well anyway.
Check everything aftermarket that you have.
The test I did was with amps I think.
You disconnect one battery terminal, connect one lead from a volt meter to the cable and the other to the empty battery post. It should read a low number if everything is ok.