Here's what I've found doing some research in the library and other places. I'd have to use a pair of 6v batteries hooked in parallel to make it 12v for my TT. But, a pair of 6v with 100AH each, hooked in parallel would only produce 100AH; not 200AH. However, one 12v at 100AH would would cost less than a pair of 6v.
But, if 6v gets more cycles than a 12v, the long term cost would be less. (So glad I didn't choose to be an accountant.)
I did a search for golf cart batteries on Amazon and found there are 12v batteries that claim to be true deep cycle batteries. Here's an example of a 12v 35AH: Mighty Max Battery 12V 35AH SLA Internal Thread Battery Kangaroo TG-31 Golf Cart Brand Product -
Amazon.com: 12V 35AH SLA Internal Thread Battery for Kangaroo TG-31 Golf Cart : Health & Household
I also found 12v 100AH batteries which are used for solar and TT applications. Example: Mighty Max Battery 12V 100AH Battery Solar Wind DEEP Cycle VRLA 12V -
https://www.amazon.com/dp/B00S2MDZFK/ref=psdc_389574011_t3_B00S1RT58C
With the growth of solar applications, I'm wondering if more manufacturers are producing 12v true deep cycle batteries at a lower price than Trojan.