Battery capacity is simply a matter of weight, which usually means size. Wet Cell lead acid batteries will have about 12 watt hours of usable energy per pound (50% cost effective point -see smartgauge.com for analysis) depending upon temperature, use profile, age, and more - each of which can make more than a 10% difference.
Get batteries that will fit in the spot you have for them from a reputable retailer who sells a lot to folks that use them like you do and stands behind what he sells.
Be very wary of the hype and myth mongering about RV batteries. Go by specifications, warranty, and cost - things that have objective measures that someone will put their money on.
The battery voltage has nothing to do with how long they last. How you use and care for them does. Generally (re NAWS FAQ), you can expect 4 to 7 years for the batteries normally used in RV service, either 6v or 12v
One man's experience does not a generality make so be very careful about anecdotes. There is a lot of very bad, incorrect, misleading, and out of context information out there so be careful and stick to objective measures with known parameters that are statistically valid and have proper backing.
The IP 9260 should do an excellent job of charging and maintaining your batteries to keep them in top shape if you let it (some people unplug for storage which is not a good thing or don't recharge promptly and so on).
Also keep in mind that batteries just don't store much energy. Minor differences in size won't make any real difference (placebo, maybe). In a TT, you are generally limited to battery banks under 250# so you have maybe 3 kWh energy available maximum. Typical household use is ten times that.
If you find yourself running your batteries below 12.0v (measured after a half hour of no signficant charge or discharge) on a routine basis, then the need is to change your energy use lifestyle, not to try to get bigger or better batteries. Running your batteries lower than this, excess heat, and improper storage maintenance are the primary ways people kill batteries.