According to the photo in the operator's manual, it would have originally had two 12v house batteries and one chassis battery. See section 6:
https://winnebagoind.com/resources/m...6Sightseer.pdf
It will have more capacity with two house batteries, either two 12v batteries in parallel or, IMHO, better yet, two 6v "golf cart" batteries in series. Your battery tray should accommodate either, but measure to be sure (golf cart batteries are taller).
If you go the 12V parallel route, it's best to replace your existing battery with two new ones. If your existing battery is new or nearly new, you're probably OK by adding an identical battery. Obviously the 6v series approach will require two new batteries.
If you're unsure about series vs. parallel, this may help:
http://www.rvforum.net/miscfiles/batterycon-1.pdf
Personally, I've always been happy with Costco, Sam's Club, or Walmart batteries but there are those who wouldn't have anything but Trojan batteries (higher quality, higher price) and, those who think lithium ion batteries are the way to go (much, much, higher price). Here's a thread on lithium ion batteries which, I think, are an unnecessary luxury for most of us:
http://www.winnieowners.com/forums/f...es-353341.html