I agree with the others that a standard TT will kill mileage due to wind resistance, more than weight. I'd like to suggest another option might be a Trail Manor, which is another variation on the Hi-Lo concept, but in current production.
As a tow vehicle, I'd second the diesel options and suggest you consider also a BMW X5 diesel or the Mercedes M class diesel or a VW Taureg diesel besides the Jeep. All have weight ratings in the 6500-7000 lb range, and should be capable of 13-15 mpg with a standard TT, more with the low profile pop-up ones.
I tow a 21 foot TT with a diesel Jeep Liberty (no longer available in the US). The trailer fully loaded is right at the 5000 lb rating of the Libby, and I average 14 mpg towing.
