There are many 'parasitic' loads on modern vehicles, letting one sit for long periods with no recharging is a sure way to a dead battery. You can put a shut off on chassis batteries, or install solar charger on the roof of sufficient size to counteract electrical loss. By the way, a battery will loose a percentage of its charge in storage, even not hooked up.
Bob & Donna
'98 Gulf Stream Sun Voyager DP being pushed by a '00 Beetle TDI