Luis, Concorde sun extender AGM is the same company that makes Lifeline AGM's which are basically the top DOG cycleable AGM battery available with the thickest positive plates.
One might surmise that AGM batteries designed for Solar applications will be happier with lesser currents compared to the run of the Mill AGM battery. However I do not know if the Sunextender brand is made any different internally than regular Lifeline AGM. It could just be a different sticker on the same battery and a higher price.
Checking the Specs it is impossible to tell as they do not list capacity at the standard 20 hour rate like Lifeline AGM but use the 24 hour rate. The weights are similar among the same size battery. I'd guess they are the same
If a 325 watt panel is your choice, then the choice is how much battery capacity to get for it.
If the solar is to be the only recharge source, then I'd say that 200 to 230 AH of AGM capacity is a good match.
Can you live on 100 to 115AH or less ?
If you somewhat regularly implement other higher amp charging sources like a well wired alternator or 40+ amp plug in charger, then one can have more AGM capacity for the solar wattage.
Capacity requirements in winter are different than in summer, as not only do shorter days mean less time for the sun to recharge, and less current during those those times, but longer nights to discharge the battery more and possibly heavier loads too, like for powering a heating pad.
I just checked the COncorde sunextender PDF and it is basically the same as Lifeline AGM, and they say for repetitive cycling to 50% or less, then a charging source of 0.2C, or 20 amps for a 100 AH battery are required. Same as Lifeline AGM.
http://www.sunxtender.com/pdfs/Sun_Xtender_Battery_Technical_Manual.pdf
Not meeting the ideal battery manufacturer recommendations is not going to cause the battery bank to fail instantly, but with such expensive batteries one should at least know what is Ideal for them, and draw a line in the sand as to how much effort one is willing to go to meet that Ideal recharge pattern.
Getting them to 100% as often as possible is likely more important than the rate at which they are charged, BUT, more solar per the capacity carried makes it more likely that both requirements are met a higher rate of recharge and a true 100% recharge too.
But obviously one does not want to take the batteries below 50% regularly either. Some people have large banks and never draw their banks below 75%, and this works fine too, but at this state of charge and higher batteries resist being recharged at high rates. So while the less deep the discharge cycle, the less abusive it is to the battery, and one has a bigger buffer for bad weather, when the weather does clear up, then too little solar does not meet the thirsty AGM requirement of 0.2C, or 20 amps per 100AH of storage, and getting high amp chargers that can meet the specs is also not a cakewalk in either the shopping or payment department.
Depleted AGMs can ask for Huge recharging currents from a well wired alternator, much more so than flooded batteries, and as such can overheat an Alternator shortening its life.
So tailoring your battery capacity to your solar wattage is wise, as long as it meets your overnight requirements without dropping below 50% each night. If other charging sources are available to recharge at higher rates and employed regularly, then less Solar wattage per battery capacity is acceptable without worry. If every morning the alternator can deliver 50 amps for 30 minutes or longer and then solar can complete the task before 1:30 PM, the batteries will be happy and live a long life.
It is the AGM batteries that get to only 95% or less day after day which will not have a respectable Lifespan, especially considering how much they cost. Flooded batteries also suffer from incomplete charging, but not to the same degree as AGM.
Most people only notice battery capacity loss when it becomes rather severe. Until that point they often unwisely proclaim everything is just fine, or as good as new. Both Lifeline and Concorde sun extender list a Conditioning procedure for when this capacity loss becomes noticeable, and it is similar to an Equalization cycle for a flooded battery, and it requires a special charger which can do the prescribed procedure and also requires constant monitoring of battery temperature.
Like a flooded battery's Equalization cycle, which is very beneficial to battery life, the LifelineAGM 'conditioning cycle is also beneficial. But also the closer the battery returns to full charge after every discharge, the less EQ or conditioning cycles are required, and the less time they will need to be administered to return the battery to its maximum remaining (not the new) capacity.
So will you regularly implement other high amp charging sources? If so then you can get away with more battery capacity.
Will your Solar charge controller allow you to set how long the battery is held at Absorption voltage? This is likely the biggest factor in how long a battery can perform/last when solar is to be the primary recharge source, on both flooded or AGM batteries, but is more important for AGM batteries, not only because they are costlier, but because they are less tolerant of not reaching full charge cycle after cycle.
And time at absorption voltage is the determinant of how close to a true 100% state of charge the batteries get.
Lifeline states than when it takes less than 0.5amps to hold their 100 AH battery at 14.4V then the battery can be considered fully charged. If the solar controller decides that 90 minutes is enough and drops to float voltage, when 2.5 hours were needed at 14.4v before this threshold was met, then the batteries will lose capacity faster.
My Bluesky 2512i solar controller with the IPN proremote allows me to set both a time or this threshhold before it reverts to the adjustable float voltage. The absorption voltage is also adjustable.
If one is going to be in places with temperature extremes, then paying extra for a charge controller with a battery temperature sensor, which will allow more Ideal voltages to be applied at temperature extremes, will pay for itself in extended battery life.
My biggest regret with my charge controller choice, was by not paying the 35$ more for the model with the option of adding temperature sensor, and my climate is rather mild.
As far as a large 325 watt panel goes, A slight concern would be aerodynamic wind loading on the vehicle it is installed upon. Perhaps the particular vehicles aerodynamics would have the slipstream press down directly in the middle of the panel, and maybee on a two lane road, when an 18 wheeler is passing you at 65MPH in the opposite direction, perhaps the down force on the panel would surpass its strength.
Also perhaps some harmonics could develop at speed. Maybee supporting the middle of the panel's glass with something firm but without sharp edges would negate any fears of severe wind loading on such a physically large panel mounted on a vehicle travelling at highway speeds.
