Just wanted to add...I also question the actual Ah's of the Everstart. Definitely something to consider when comparing to other brands.
You're not really getting what you pay for as advertised. But, then again, we've both had these batteries with satisfactory results.
6V - Why not give them a try this time around? I have Interstate GC2-XHD-UT batt's and I am thoroughly happy. You can't beat the discharge and cycling capability of 6V's. The particular batts I have aren't all that more expensive either. If you shop around, they can be had for less than $300. Even less if you find an online deal with free shipping, no sales tax and a coupon code.
Since you have the room for 3, you could buy two 6V's and a 12V. Keep the 12 as a back-up in case one of the 6V's die. I realize that's more money and probably not what you want to do. Just an option--an option I would like to have.