You've got a point there. Trojan makes some good 12V deep cells that are just short of 6V capacities, so choices are out there. I think they just Matmay not be as commonly stocked add the 6 volts. The good thing is that either of them can work!
2016 Jayco Eagle HT 29.5BHDS (ordered 12/30/15, delivered 3/8/16)
2015 F-350 crew cab, short bed, 6.7L PSD, Pullrite Superglide 3300 hitch