On our '08 GMC Sierra 1500 with 5.3 and 4-speed auto, cruise control was the pits on even minor upgrades. The trans would shift into "passing gear" and stayed there even after cresting. I learned that if I left it in cruise, but accelerated with the gas pedal when starting up a grade, it helped from over revving. But if there was a memory lapse, it went into the ridiculous high revs. This TV has the trailer tow button on the shift lever. It was pretty annoying to drive.
We traded for a '13 Silverado with 5.3 and 6-speed auto and it made a world of difference. We were reluctant to go 6 speeds because of some bad reviews. Driving on normal hills, the transmission behaves. On steeper upgrades I may use the gas pedal along with cruise or take it off.This also has the trailer tow button on the shift lever.
What I have noticed, that using cruise, the engine is revving lower and gas mileage improves . For instance, on a quasi level roadway holding the gas pedal steady at say 65 mph, then actuating the cruise and taking the foot off the pedal, it maintains the set speed, but the rpm reduces about 500 rpms and the instance economy increases. I have noticed that if not using cruise, it seems difficult to maintain a steady speed. And I have found that, once the cruise is set and deactivated to slow down, it is wise to get back to speed with the pedal before engaging cruise to prevent a downshift. Example: cruise set at 65 mph engaged at 55 mph will cause the trans to try to get to the set speed rapidly, downshifting several gears.
These results are from towing the same 7,000 lb GVW cargo trailer fully loaded. MPG on either TV ranged from 9-11 MPG, depending on towing conditions over a distance of 1,700 miles between Florida and Wisconsin.