I would suggest you add up the amp hours available to you with the
batteries from local suppliers. Some times, I think the drive battery is
not a deep cycle type, but a high cranking amp battery, with less amp
hour rating. The two six volt batteries are deep cycle and produce more
amp hours than a 12volt normally does.
I just replaced one 12volt Deep Cycle with two Trojan AGM 6volt to
improve my amp hours. A little costly but actually they are safer as
they do not let off gas. They are also shippable by UPS.

http://www.ebatteriestogo.com/RV_Batteries.htm

As the Auld Mekanik told me: "If'n it ain't broke, don' fix it!"

If you can find 2x12V Deep Cycle batteries that, combined, provide a higher
number of Ampere-Hours than your 2x6V [probably Golf Cart] batteries AND
cost less then go for it. <G>

Unless you (or a relative/inlaw/spouse) can get a major employee discount on
12V batteries but none on 6V batteries you'll save a bunch of ca$h by going
with your existing setup.

Stay with what works:  What you have now.

With 6V golf car batteries available at Sam's Club for ~$50, you can't get a
bigger "bang" for your buck.

Thanks to all that replied.  You gave me exactly what I was looking
for.  I'll compare the amp hour of the batteries and go with the best.
I will probably be the 6v setup.

Thanks.

Dave.