Factory turbo setups (on gas engines anyway) are usually engineered to provide boost for short bursts, like on acceleration, not for long pulls on the hwy. I owned a 4 cyl T-Bird with a turbo & a 5 speed manual and would marvel at how well matched the engine power --> turbo boost --> gear ratios --> vehicle weight were to work well in any situation. There was no boost on normal hwy cruising. On hills it would see some boost, but not full on like acceleration. Towing a trailer voids the warranty because of this.
The point I'm trying to make is, if you rely on a turbo for long pulls on the hwy, you're going to put a lot of extra stress on your engine for long periods and it's not likely to end well.