I agree with family wagon..... while you technically can use them, I would only do it for the short term, or as backup power.
You'd be a lot better off investing that money in deep cycle batteries, my personal preference is for 6v golf cart batteries since the amp hour rating is higher than you will find with 12v batteries. Granted you'll need more of them but the cost per amp hour will be less if you choose wisely.
I'll work up a cost per amp hour for some different batteries, then you can use that to choose something in your budget that will work.
EDIT: A Google Sheets workbook showing $/AH based off info found at batteriesplus.com. As of this edit it only has Duracell Ultra 6v and 12v batteries listed but that will be enough to show the cost difference between 6v and 12v.
One other thing... for the 6v the $/AH is calculated using TWO batteries in series to get 12v.