Beg, borrow or steal a clamp on DC ammeter and do this experiment:
Clamp it around the DC negative wire and measure the current and record it. Then one by one remove each fuse in the converter panel and record the change. Replace the fuse and go to the next. Then if there is a breaker for the generator starter use the meter to record its current. Then look for a #2 gauge or similar red wire connected to the battery but not a breaker which goes to the BIM and use the clamp on meter to record the current.
Once you have done all of this report the results.
Other than simple stuff like the use/store switch, it will take some investigation to get to the bottom of this.
FWIW my 2021 Axis draws 0.7 amps with everything off except the tankless water heater and 0.5 amps when i turn it off at its outside switch. With normal use of lights, device charging, etc. I use 25 amp hours every 24 hours and can go four days on my two 100 Ah AGM batteries.