Are they charging you for charging & discharging?
I'm going to pester you now a little bit: if you had an
LED battery pack monitor like these ones, you'd already know which were your best & worst batteries in terms of both "stiffness" and capacity. Having one in your own vehicle is like having constant rolling load testing under the conditions you actually use them.
In terms of instrumentation, I'd say it's actually more important than the volts or amps gauges.
Put it on your to-do list!
Still, it'll be interesting to know what they discover.