I, too, employ the Scan Gauge II to monitor my transmission temps. It is a very helpful gauge that provides an array of data via your OBDII connector. With respect to monitoring the transmission temps, you need to enter codes specific to your TV. See this link for the type of commands and vehicles that the gauge supports. http://www.scangauge.com/support/x-gauge-commands/
Although everyone's transmission temps will differ due to rig set-up, climate, altitude, and the like, I am posting a chart that shows the number of miles your transmission fluid "life" is reduced (due to oxidation) as your fluid temp increases. The data was prepared by the California Dept of Forestry and Fire Protection.
For those of you who insist (or are considering) on towing large TTs with 1/2 TVs, even if your rig is within "spec," it is recommend that you consider these other types of "non-spec" data points, too.
2004 Suburban 2500 LT 8.1L/4.10/4x4
Putnam XDR Class V, Equalizer 1400#
2006 Jayco Eagle 264BHS
Iota DLS-55, Progressive Industries EMS-HW30C, Trimetric TM-2025