Good advice from twinboat. There is nothing special about an 8D battery - it's just a big package that is equivalent a multiple smaller case-size batteries. A typical 8D stores about 240 amp-hours, roughly the same as two of those GC2 6v's or a pair of Group 31 12v batteries.
Former owner of 2004 American Tradition and several other RVs
Home is in the Ocala Nat'l Forest near Ocala, FL
Summers in Black Mountain, NC