question about starting and battery voltage

[sleepstar]

recently began having some issues starting. haven't had a no start yet, but cranking is getting real slow. i thought it might be the battery, took it to autozone and the worker said it was "70% charged but good." this is after driving there for 10 min at a steady 30mph and no acc on at all. they said they couldnt test the alt because it wasn't 100%. she suggested letting it idle for 5 min to charge it, but i dont think the worker understood that would be no different than driving it there since its a diesel and used zero battery on the way there.

my voltmeter gives the following readings:

at rest: 12.2V
glowing: 11.3V
right after glowing: 11.95V
starting: varies, low as 9V to high 10s
idle: 13.14V after starting... then after running for about 5 min 14.2V

battery is MB , 3 years old, alt is reman bosch 5 years old, starter is reman bosch about 2 months old... new ground strap, all connections cleaned