Six weeks ago I decided to replace our four-year old coach batteries in our 2015 Winnebago Forza.
I went from four batteries to five.
Besides the extra battery you need a hold-down, a negative cable and a positive cable.
Someone else said you could pick up the cables at any parts store. Well, that's not gonna happen.
To add another battery to my Forza needed battery cables 9 inches long, 3/0 gauge. I never found any available "off the shelf" for even close to a reasonable price. I settled for 12 inches long and 4/0 gauge at $35 for the pair.
Remember that 4/0 is heavier than 3/0 and that 4 gauge is tiny by comparison.