Assuming the 6 6 VDC batteries are the common golf cart batteries (around 220 amp-hrs @ 6 VDC), your current battery bank is 660 amp-hrs @ 12 VDC.
Yes, you could choose to go with the same amp-hrs in 12 VDC (3 8D's), though in general quality 12 VDC batteries are more expensive per amp-hr than golf cart/6 VDC batteries.
As far as lower amp rating battery bank of either 6 or 12 volt batteries, the answer depends on how you will be using them. My personal test is that my battery bank run everything I would expect to run overnight (say 12 hours) without needing a recharge and without discharging the batteries more than 50%.
Clearly the largest draw would be items using inverter power.
You will have to tell us what size inverter you have and what electrical needs you expect for us to be able to suggest reducing the battery size (and the same answer would apply to either 6 or 12 VDC batteries). In fact, just think of two 6 VDC batteries wired in series as a "12 VDC battery in two pieces".
Brett and Dianne Wolfe
2003 Alpine 38'
Moderator, FMCA Forums
Chairman, FMCA Technical Advisory Committee
Member, FMCA Long-Range and Development Committee 2007-2009 and 2014-
Moderator, http://www.dieselrvclub.org/(FMCA chapter)