Whatever size it came with, which should be on the existing battery label. It will have a case size designated by a BCI Group #, e.g. Group 27 or Group 65, and a CCA rating, e.g. CCA 750 Amps. You want the same case size and same or greater CCA rating. Brand is not terribly important as long as the specs are right. Most batteries are private-label versions of standard battery designs produced by one of the Big Four US battery factories.
I'm guessing you have a Ford F53 chassis (V10 engine) under your Winnie. If so, it probably requires a Group 65 case size and a minimum CCA of 750. A premium grade of the same battery type would have an 850 CCA. Wither one will be plenty.
At age 6, your house batteries may still be ok or they may be limping, depending on how they have been used and maintained. If you mostly use electric hook-ups, it's perhaps not much concern yet, but be prepared for house battery problems before too long.