If you want to tow a heavy boat, get yourself a strong truck.
If you get yourself a strong truck, don't expect real good gas mileage.
If your worried about poor gas mileage, get yourself a small boat that can be towed with a small truck.
You can't have everything boys.
I bought myself a boat that's around 3500 pounds, so I also bought myself an F-150 supercrew with a 5.4 motor to tow it with. It's a stong truck and does a great job of towing the boat, it also stops very well and has plenty of room for carrying my gear.
It's OK on fuel consumption but not good, but I knew that when I bought it and as such, I don't worry about it.
It is what it is.