What type slide mechanism, hydraulic, gear, etc.? Have you tried disconnecting engine battery while in storage to see if there really is a connection between slide moving and battery going dead. How old is the engine battery? The ECM (engine control module) will drain the engine battery eventually while in storage. How long that takes depends on the condition of the battery.
Hank & Lynda
2003 Winnebago Adventurer 35U, Workhorse W22