To track it down with a multimeter, you can pull one fuse at a time and see which circuit is using the current when everything is 'off.' In addition, usually the CO and LP detectors are hard wired to the batteries and bypass the fuse panel completely. Wireless door locks, security systems, etc. also can use energy waiting for a signal. Good luck, let us know what you find. On mine, my chassis batteries do fine after using the disconnect. On the house batteries, even with disconnect off batteries will drain over time. Installing a knife switch on the battery terminal has solved that issue for storage purposes.
Bob & Donna
'98 Gulf Stream Sun Voyager DP being pushed by a '00 Beetle TDI