Help! Slow Battery Discharge

You're getting your AMPS and VOLTS mixed up. Amperage is the amount of current flow which is what you need to measure. Put your meter in DC current mode in series with the positive side of the battery and read the current (measured in amps or milliamps). It should read "zero" amps with everything off, doors closed etc. If it shows a current draw you'll need to start pulling fuses to find out which circuit is pulling power.