3 weeks is a normal draw down time because of the memory draw by the engine module, transmission module and radio memory.
Battery disconnect is one way to prevent if you can't plug in and charge.
Solar charger, will help if stored outside and weather permitting.
Even a single light, left on would draw battery down in 3>4 days, so if the draw takes weeks , 99% sure it's just the memory of those modules.
99DSDP 3884, Freightliner, XC, CAT 3126B, 300 HP /ALLISON 3060
2000 Caravan toad, Remco & Blue Ox.