You have it right. I'm not used to meters that you could put a short (fused) across the "normal" leads by rotating the selector to a current range. It is possible that the low range fuse is blown. Even the act of putting the meter in series could blow that likely 1/2 amp fuse.

With the meter in series with the terminal, will the interior lights light? If not, then the meter fuse is likely blown. Move on to the 10 amp range - black meter lead in center, red in left. Same deal, meter in series. Make sure lights in car light. Meter in "A" mode should show a few amps. Now do what you need to to get everything to shut down while you have the meter in series. If you read like 3 amps with the interior lights on- a guess - when it settles down, it should show "quiet" load. Should be less than maybe 200 ma.

In any event, if you get a valid current, you can calculate how long it will take to kill the battery from full charge. Battery is supposed to be 80 AH, but use 60 since multiple discharges probably have reduced capacity. use 60 divided by current read. So, if you read 1 amp, it's 60/1 - 60 hours to dead. If 100ma, then 60/.1 or 600 hours to dead or 25 days - 600 hours / 24 hours in a day.