The WFCO is a good converter till it's not. I would not replace it till I had to. Same of several other makes. My Progressive Dynamics 9180+wizard is, near as as I can tell the BEST converter.. till it failed. .now it's replaced with a refurb :).
As for the size of the batteries. That is a function of total watts stored. (amp hours times volts)
Some numbers In all cases I'm talkin 12 volt battery packs
Group 24 about 75 AH
Group 27/29 just shy/over 100 AH
Group 31 About 120-130
GC-2 pairs (Two 6 volt) 220 Amp hours
So each six volt battery is between a 29 and a 31 in weight and total volume..
Plus they are a bit taller so you don't need to water as often
And yes 1 12 volt (likely a 29) to a pair of GC-2 gets you from about 20-25 usable amp; hours to 100-115 That's a SERIOUS upgrade.
