A "shade tree" ammeter is a 12 V bulb.
Connect one side of the bulb to a battery post; connect the other side of the bulb to the cable that would normally go on that post. The cable to the other battery post needs to be connected normally.
IF.....the bulb lights bright for a few seconds and then goes dark, you have nothing to worry about.
IF....the bulb continues to glow for a LONG time, you have some phantom leakage that needs to be tracked down.
I think it's possible that you have some things like smoke, fire, CO, propane detectors that do not have a central fuse.