Bohemian, you can get more Ah from 2 6V GC-2's or T-105's (225 Ah) for around 125 lbs. They are true deep cycle batteries and not Hybrids. I have 4 6V batteries @ 60+ lbs each, and like Bob said, I can't imagine trying to get a 135 lb battery in there.
All lead acid batteries use "2V" internal cells. Thus the difference between 2-6V batteries and 1-12V battery from he same manufacturer and series of the same net capacity is the 2-6V batteries have extra casing, weight, and cabling. The same for putting multiple 12V batteries in parallel vs one larger higher capacity battery.
The differences you quote have much more to due with intended use, design and quality of manufacturing.
Technically, comparing 2-6V vs 1-12V from the same manufacturer and same deep cycle series of batteries, the 1-12V should have greater capacity per lb. and per volume and be more reliable, as is observed by technical studies.
The weight and handling issues are of real concern.