I suppose how the battery disconnect was installed and where could be an issue. Try physically removing the cables from the battery and see how that goes. If you have good voltage after a week it could mean that the battery disconnect is not disconnecting your propane leak detector, etc., or there could be a short somewhere.
Some people also install a single pole switch to disconnect the propane leak detector but it could be dangerous if you forget to turn it back on while in use.
2016 Ford F-150 4x4 Supercab XLT EcoBoost
2012 Salem Cruise Lite 221RBXL, Andersen No Sway WDH, (2) Honda EU2000i's