Battery help.. question for the battery experts

just finished my first season of camping in my new TT.. really really dumb question on the battery.. I purchased TT brand new in June and it came with a size 24 battery, around the same time that I purchased my neighbor gave me a brand spanking new size 24 battery that he purchased the previous week for his camper but ended up not using.. so I now have two identical batteries.. problem is, I never dry camped this season.. every site I went too had power.. so this may sound like dumb question but what in the world do I do with a brand new unused battery? should I just hook up both batteries and use both? I know how to hook up both, but seems silly to hook up both batteries if I don't even use one battery, or is there a way to store a battery long-term once its been charged?????
the only reason im even considering hooking up both is so that one battery doesn't just sit on my front porch rotting away because its not being used... its brand new and seems silly to just let it sit there...
my second question is, for this winter, I have the batterys in my basement and I have a small 1.5 amp charger. both batterys are fully charged and reading about 12.7 volts with a voltmeter.. I check batterys about once per week with the voltmeter, at what point do I want to hook up the charger to trickle charge it? I don't want to drain them that far because im sure 1.5 amps wont do much with size 24 deep cycle....thanks for the help...