As a former truck-driver, I can tell you you don't want to be behind big trucks! First of all, if you're close enough to truly draft, you're too close to stop if the truck has to stop suddenly. Don't think for a second you're reaction time is quick enough! True drafting of a big truck is within 20 feet or less. Also, as mentioned before, if the driver sees an obstacle in the road they will straddle it if they see it in time and you won't see it until it's too late! If they don't see it in time, all kinds of debris can be hitting you from everywhere as it gets shattered by the tires! Another little tidbit you won't hear much about is a certain breed of drivers that drive as teams. They have an access panel in the sleeper floor and do their business through that rather than stop. You really don't want to be behind them at that point!
All that being said, many, many years ago when I was young, I drafted a truck for about 20 miles on the interstate at about 10 feet behind. It was after dark in the summer. I noticed a few tiny drops of water on my windshield but, it had rained earlier so, I figured it was off the road. After I realized I should probably get out from behind the truck, I cut into the left lane to pass and discovered I was in the middle of a massive rainstorm! While behind the truck, I had no idea of the storm and also, I was barely above idle-throttle to maintain about 70mph! It's very effective but, not worth the risk!