I've towed a lot with my high mileage Tahoe at and we'll above the rated tow capacity, albeit with a rebuilt trans. Here are a couple notes:
The advice to keep it in 3rd is paramount. Fuel is cheap compared to trans rebuild.
The factory engine temp gauge is not 100% accurate. Will still read ~200* until around 220* actual temp. Monitor engine temps on grades VIA OBD2. Otherwise, don't fear the RPM.
I just recommended against it in another thread, but for towing a TruCooler 40k is a nice tranny cooler that will replace your factory one with relative ease. For the cost of one on eBay, might be worth installing for the trip and going back to a smaller cooler for regular driving.
You should be fine at 70mph. I think my longest haul with a heavy load behind my Tahoe was Indianapolis to Bristol, VA with around 9,000-10,000#. My Tahoe has 3.73 gears and had 33" tires at the time, I maintained a happy 75 most of the trip. Remember, highway driving is what's easiest on a vehicle. Speed isn't the enemy, lots of shifting is (as mentioned).
Don't go overboard on maintenance before the trip. Oil change, grease the ball joints, make sure you've got gear oil in the rear end - sure. Tranny service is 50/50. Has it been done before? How long? If you have a filter/fluid change done, have them drop the valve body and make sure none of the check balls have beat their way into the separator plate, and put a shift kit in while you're in there and block off the PWM solenoid. Honestly though, if the trans was acting right I probably wouldn't touch it and just make sure it stays cool. Definitely do not have it flushed. Mostly make sure everything is working as it should and if it is don't mess with it just before a trip.
*If* you have a tune done, have them tune out the Torque Converter PWM. That's your #2 weak link that will take out the trans.