is there a light bulb in the storage space?....if so then get one of those adapters from home depot that has a plug and allows the bulb to screw in and then plug a 6 foot extension cord to a power strip and then plug in the necessary amount of Battery Tender's......if the owner of the storage space says anything...then remove it all....but if I were you....that'd be my reccomendation....also consider the types of batteries there are out there instead of the liquid lead cell...there are Gel Cell and AGM batteries.....this company though seels a great charger with their batteries as an option and I got my most recent battery from them and a charger and I have used it to charge a couple of batteries so far and it goes to auto trickle!
http://www.westcobattery.com/index.html
http://www.westcobattery.com/index.html