I often get flamed for this but you don't have 6 Volt Batteries IN the motor home
(They are only six volt when they are OUT of the motor home)
YOu have 12 volt batteries which have been split in two for ease of handling.
Each pair of six volt when properly installed make one big (Size 4D or 8D) 12 volt battery.
So why not just use the bigger battery? Two answers
One the GC-2 is the single most made Lead Acid storage battery.. That makes them cheaper.. (Golf Courses order 'em by the pallet load)
Second.. Check out the LBS column on the 4D and 8D (Oh my Aching muscle specials)