Based on your other thread you mentioned a breaker panel as the WFCO converter part number. My searched turned up that the part number is only the 30/50 amp breaker panel, but compatible with a WFCO 9800 series converter. I still think you should check to see if the fan is working, as your problem sounds like thermal protection with it working intermitently.
As for autozone or advanced auto, they use the same tests used by shops. They are pretty accurate on the battery being good or not and can even dected how much CCA is left in the batteries. Only the big heavy load testers (not the handheld) can detect aH or Amp hour capacity. But from what I found, CCA capacity even in 6v, when they get below half, the battery is done.