I had same problem was the t-stat

 

Not likely your problem but my '03 (purchased new) would only read around 180 while towing and if cold outside would darn near register at all after ideling for a while. After I had the faulty fan clutch replaced under warranty it was found that the temp gauge needed to be recalibrated. After recalibration it consistently reads about 210 before the t-stat opens and around 190 afterwards. Granted, that's been about 8 years ago but it still reads the same.

Just my .02

 

i beleieve the tool your are looking for to test the temp that your antifreeze with hold up to (freezing point) is call a hydrometer, and yes they are still around. quite common in most parts stores