At the beginning of Jan i drove a 28ft uhaul towing a 16ft trailer loaded with a Jimmy on it from Houston to Sacramento.
The mythbusters show is true, it said that around 100ft you get a 10% improvement.
What i did, was pretty much stayed behind truckers that were doing 65-70 and stayed a car length behind (or the trucklength actually, about 40 ft behind him.) This still gave us an improvement in gas mileage, and also allowed me to relax because it was REALLY windy and the truck would break the wind for me so it wasnt so bad.
Also, at nighttime and in a heavy fog (like when we got to california) it allows you to see in front of you better because you KNOW you can stop faster then the 18 wheeler.
Good luck. //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif