Sixto,

That will drain most batteries over a night or a day. Drains then recharges I believe used to weaken the old lead plate batteries. That can cause a plate to fall over and short.

I agree you need a voltmeter / ammeter. Running, you should have 13.5-14.5 volts. If not your charging system isn't going to keep up.

To find the drain you are down to troubleshooting by process of elimination. Hook the ammeter in series with one of your battery cables and keep disconnection fuses until the drain goes away. That is likely the circuit.

The other hassle could be the drain comes from a short caused by some previous event. Like maybe some of that environmentally friendly wiring in the engine compartment.

Years ago a friend had a 74 Chrysler New Yorker. The Chrysler version of the 140. More wire than you can imagine. Each door had 2 lights, a lighter, power windows and locks. He had a short we couldn't find. We tore that thing apart. It turned out it had been in an accident and dash was removed. When they put it back in a wire was rubbing over a metal support bracket. Over time the insulation wore off and the short started. Hope your find is much easier.

Chuck