Costco now carrying 6v Golf Cart batteries.
 

220 amp hour 6v batteries. Actually, they are rated at 75amp for 100 minutes. $78 each. 63 pounds each. $9 core fee.

They appear almost identical to the Trojan T105.

I haven't looked at their spec sheet, but they likely deliver ~125 AH at the 1 hr rate, 147 AH at the 2 hr rate, with an internal resistance of about 0.004 Ohms. Perfect for an inexpensive conversion.

Similarly, you can get Energizer E-GC2 6V at Sam's Club. They're made by JCI, and you can email them for a full data sheet, including cycle life vs DoD.

Is there an advantage with 6v over something like the Trojan SCS225 12v battery? It's 66lbs. 2 of those would seem to be equal to 4 of the 6v ones.

looks like the 6v pair would be $50 cheaper.