I understand your situation, I have been there and it is frustrating.
I have fought tax valuation with mixed results. The advice I would have is to definitely fight it, but don't expect to "win". I would expect to be able to push the dial to some extent your direction, which is worth doing, but they aren't likely to drop your level to what you believe is right.
The key is persistence, the system seams almost built to wear you down. It takes phone calls, paperwork, following up and pushing things along. Finding out what might get them to make changes and doing it. Nothing really hard, just busywork for the most part, and fighting back against their "logic" with your own logic.
But the benefits compound over time, definitely worth the fight.