Just do like people do with a car in prolonged storage - physically disconnect all the loads from battery. Radio clock on standby, CO or propane detector, some odd forgotten light, short in old wiring, who knows...
Don't believe when they say "dead" until you test it yourself. More than a year ago people replacing a fuel pump in my truck told me that battery needed immediate replacement - as usual they wrote a looong list of deficiencies after fixing the one thing that needed to be fixed. Went to a friend who works in truck service - battery tested OK. Starts without problems still now.