Just a comment to the posters about vehicle size/ type. Take a good look at the new trucks on the market. Most all come with a variable V8 which allows the engine to run on 8,6,4 cylinders as needed. The transmissions now are mostly 6 speed. Factory towing packages allow the transmission to use the gear limits to a better advantage than the average driver can drive. The transmission will downshift as needed based on your speed like a diesel exhaust brake.
I tow a #7000 lb. trailer with a 2013 Chev 1500 5.3 ltr. with factory towing package. Have had little to no problems on the road. Have towed from Phoenix,az to flagstaff, az no up hill or down hill problems.
Don't want to be cruel but a lot of you commenting here are driving older vehicles and have not done your homework on the new technology.
I am a retired long haul truck driver and yes I do understand wt and balance hills and downgrades. Try a downhill on I70 in Colorado with a gross weight of 80,000 lbs.