Grab any sort of small 12 volt automotive light bulb. A test light works great if you have one. Remove a cable from the battery (either one) and somehow wire the light between the batt terminal and the cable you removed. If the bulb lights you have a draw.

At this point you can start pulling fuses one at a time, and putting them back, until you find one that causes the bulb to go out. When that happens you have at least found the circuit that has the draw.