i agree with boB about the series string issues with agm's and not to ever centertap the string for anything, ever!
i did it once on a 24volt string of agms, 4x6volts, tapped 12volts off for a temporary test and got distracted, called away and left the load attached..took me over a week of charge/drain/charge/equalize carefully, wash rinse repeat to get the pair back into shape,,, i didn't think i ever would get that done..
the way it was, even after a few recovery cycles i would still get a .3-.5 volt difference between the battery halves of the bank, the untapped top half would keep going well over 7.8 volts while the lower
would barely make 6.2 and the charger would then taper off. 7.8 volts on the upper batteries really makes a lot of heat, swells them up and is generally a bad deal all together, of course the 6.2 volts on the lower half batteries means they are not getting to even half charge which over time only gets worse in a hurry.
my next string will have a plus one strategy, meaning i will have one spare battery that gets rotated through the string, the spare can be monitored, charged and equalized if necessary to keep it into shape, and then swapped for a weak sister regularly.
i did the same as maryalana, i went to ebay and bought a 24/12 dc/dc converter, that allows me to have 6amps of 12volts for testing if needed. i also bought a 48/12 converter for when i am testing with a 48volt string... i think i paid about 20 bucks each for them, used surplus units,,, they seem to work fine and i suspect will give good service as long as i don't overload them, although i don't think the efficiency is as good as some of the higher dollar offerings. this i don't know for sure as i have not tested watts in vs watts out yet on either of them.
as for the series vs parallel argument, i suppose there are reasons why each is preferable under differing conditions. as long as one is keeping a periodic eye on the battery conditions, water levels and specific gravity of each cell, either can be made to work out just fine.
bob g