Crossing over a 6v flooded lead acid battery

[Howie]

It all depends on the motor specs and condition of the wiring in the unit. If you knew the hp of the motor you could calculate the battery draw.

Yep, motor draw.  Sounds like you might be OK with the tenders and fresh batteries with such low usage.  Our fork lift (7000 lbs with a 3000 lb 48 volt battery) would load or unload 20-30 American V8s with a battery that was about close to dead, specific gravity in most cells less than 1.50.  Try it and see how it works out.  In a pinch you could speed things up with this not so smart not so expensive charger  http://www.amazon.com/Schumacher-SE-5212A-Automatic-Handheld-Battery/dp/B0009IBJCQ/ref=sr_1_3?s=automotive&ie=UTF8&qid=1421185981&sr=1-3&keywords=automotive+battery+chargers