Those are pretty typical for 40-60* temps. Do you have the optional "trailer towing" package that includes a transmission cooler? You would be able to see it through the grille, near the center, right behind the grille.
There is no thermostat on the transmission cooling, which is actually done in the right hand side radiator tank by the cooling water. That is the cool water, return, side of the radiator so when it is cool out the water there will not be very warm because of the water thermostat being mostly closed to keep the engine warm.
A thermostat can be added to keep the transmission warmer in cold weather, but it is rarely necessary unless you see well below freezing weather a lot. Thermostats are of more benefit if you have the add on transmission cooler, as they can make it run very cool in cold weather.
Most of us have found that the 4 speed transmissions in the Chevies are much more likely to have problems with getting too hot, rather than too cold, especially on steep climbs in 2nd or 3rd gear. Highway driving in 4th gear like you were doing will show the trans running quite cool in most cases because the torque converter is locked up and there is lots of cooling air. You would see the temp come up quickly with some stop and go driving in town, even in the cool weather.
You will see temps higher than the normally recommended 170*F that we have always heard as the best high limit. It has to do where the temp is sensed in the transmission, and the fact they just let them run hotter in these transmissions. Without added cooling, most will see 210-220*, or even more, fairly regularly if you are in hills, traffic, heat. Normally, you won't see the transmission temp go over 200* until the water temp starts to creep up off of the 195* normal temperature, and then they will climb together.
Your 2003 came with Dexron IV transmission fluid, which is non synthetic. GM switched to Dexron VI in 2007, for the van transmissions, which is a full synthetic and said to be safe for about 20* higher temperatures than the Dexron IV. If you are due for a transmission fluid change, it would be a definitely good idea to get the transmission fully flushed and replaced with the Dexron VI.
We have had a lot of discussions on keeping the Chevies cool in the mountains and heat on this forum and most of us have found it isn't particularly easy.