Yes, it will keep them at the float charge, no problem. Battery Tender Jr. will also do the bulk, absorbtion, and float, so that you can hook it up to a battery that isn't up to full charge.
Vehicle batteries don't have the problem of needed the electrolyte stirred up by an equalizing charge (an intentional overcharge for an hour or 2), which makes the gassing bubbles come up, and theoretically 'stirs' the liquid, as well as requiring water to be added later. Some solar batteries can weigh more than I can lift, so you're supposed to overcharge them to mix up the electrolyte when the individual cells get to be more than a tiny bit different.
Vehicle batteries get vibrated enough to not need this overcharge, and your storage time will be without any deep discharge cycles if you keep the BT jr hooked up.
The battery tender Jr. will keep your battery from freezing, too. If you hook up a light or a fan to the battery while it's in storage, just leave the BT jr hooked up and it will go to the correct recharge voltage, then go back into float when it's charged.
