How much does your Sprinter actually weigh?
I have a Jeep Libby with the same 2.8 liter 188 hp diesel engine, and my TV and TT combined weight is pretty close to 10,000 lbs fully loaded and ready for bear. Other than a propensity to overheat, it pulls hills fine. I was easily able to keep up with traffic over the Rockies on I-70 west of Denver a couple years ago. I'd hope that the cooling system on your Sprinter was better than on my Jeep, but the engine itself should be able to handle it.
As for the performance, if you double the weight, you will obviously be slower off the line. I get 30 mpg without the TT, 14 with it. What do you get?