What year is your Tahoe? I don't think GM put a transmission temp gauge in any of them but could be wrong. I swapped out my cluster with one that has trans temp. SUPER helpful.
FYI ~ On fluid, others may not agree with me... However, I don't change it out that frequently anymore. I used to do every 20,000 miles but felt was a waste in hindsight. Get a clean white paper towel, wipe dipstick on it, fluid should be pink / red on the dipstick. If not, then you should change the fluid. Otherwise, you can change the filter and reuse the fluid. It lasts MUCH longer than people realize, unless you run the transmission too hot.