So you will have 2x 6v as #1 and a single 29 12v as #2? As long as the old 12v is in good condition, I see no problem.
Are you aware that two of the 12v 29's provide about the same power (amp hours) as 2 x 6v GC2's? I'm not trying to talk you out of the 6v's, but you could get pretty much the same effect by adding another 29 12v to your present set-up.
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