My brother towed a 340 Sea Ray (about 13k lbs) with a V6 Ford Ranger once. Had to block the suspension but it was done.*
Get a 7.3 powerstroke and slap a natural gas kit on it. More power than anything mentioned in here and it will get great mileage with the kit. Just don't put huge wheels and tires on it and throw away your MPGs.
* - No hills were involved in this journey.
I'll be in my bunk.
Let me get this straight: You want a more fuel efficient vehicle to tow a boat. A gasoline powered boat. A gasoline powered boat that costs hundreds of dollars to fill up every time you take it out. What is the boat fuel usage cost compared to the car usage cost? Seems to me, that if your wife really cared about spending less on fuel costs, you'd be looking at a SAILBOAT!!!
(PS: I still lust after your old 2500 'burban)