I upgraded the battery in my fifthwheel RV, replacing the existing size 27 12 volt battery with two 6 volt golf cart batteries connected in series. I needed a new battery box that would hold the new batteries.
In my area, battery boxes designed to hold two golf cart batteries cost anywhere from $75 to $85. I figured there must be a less expensive solution, so I searched the local Wal-Mart, Target and K-Mart stores and found a $15 cooler that was just the right size. Actually, the width and depth were right on the money but the length was about 6 inches too long. I took advantage of this and used part of the extra space to store my distilled water that I use to top off the batteries.
My batteries are the Wal-Mart (Sam's Club) golf cart batteries. The sizes and specifications are almost identical to the Trojan T105 but the Wal-Mart batteries are about $30 cheaper. I drilled 1/2 inch holes in the side of the cooler to run the battery cables. I also cut a hole in the top cover and ran the existing vent tube into this hole. I also secured the cooler to the floor of the foward compartment of my RV by running four sheet metal screws through the bottom of the cooler.