That's a very good number for a 24 foot trailer but it will likely go higher on long hills or bucking headwinds.
Does the truck have 3.73 or 4.10:1 axle ratios? My testing has shown about a 25-30 degree difference for each ratio change (e.g. 3.0:1 to 4.0:1, with the taller gears runner hotter.
Trans temp is going to be high when towing and that's why you have to change the fluid more often if you tow a lot. It's time at temp that causes the oxidation and that's why a good syn transmission fluid is worth having because the oxidation rate is slower. Also look nearby at the conversations about the Magnefine filters, you are a perfect candidate!
Your engine oil usually runs around 200F and it's not unusual to sustain that much on the transmission fluid over a long day pulling. Trans temp can be measured in several places... I don't know where the GM sensor is located. By that number, I suspect the pan. That is the most stable place to measure temp. If measuring the converter temp, (usually in the line out to the cooler) it will run lots higher and be very active going up with loads and when the converter is unlocked. It usually runs 50-100 degrees hotter than the pan temp, depending on how much slippage is going on at the time.
My realistic rules of thumb are that if your truck sustains up to about 212F while towing, you are generally OK. If you sustain more than that, assuming you are not overloaded, you should consider upgrading the trans cooling. Short spikes of up to 250F are really nothing to worry about. By short, I mean in the 15-30 minute range. I start to worry running hours at higher temps because that can really start to work on the oil. You really should not see temps much above 250 for anything but very brief periods and preferably not at all.
Some people prefer lower numbers. To me it comes down to how much you tow. The numbers above reflect what I've seen in a number of trucks in the real world and I've had sensors hooked up to multiple places on the trans to monitor several parameters all at once. If you are a "Snow-bird" type person or haul loads for a living, it makes more sense to spend lots more money on coolers and such. Recreationally, it's much easier and cheaper (and no more harmful) to monitor your gauge and take appropriate steps when you see high temps (again this assumes you are not overloaded). That usually comes down to dropping down a gear. If I see 250 in 3rd on a long grade and it looks like a long climb, I drop it into second and the oil temp drops 30-40 degrees. The engine revs a little more and you lose some speed, but the load is lessened on the trans becuase of the gearing advantage and you cooling system is working a bit harder (more fan and more water through the radiator). If you are a "dang-the-torpedoes" type and simply must charge up hills at the maximum rate possible, then you are more in need of cooling mods (and can I be your mechanic... I could use a payraise ( : < ).
