Thing to remember. Towing RVs, and MPG do not fit into the same sentence. It ain't going to happen. UNLESS you go against your criteria, and get a popup, or Trail Manor. The wind resistance of a TT, or 5er is the determining factor in MPG.
For the size TT you are talking about. Just about any F150, or 1500 series truck from the top there manufacturers will do what you want. As will a Full size SUV.
For lite weight. You should be looking at brands like APEX, Among the lightest TT on the market. Yet have all the bells, and whistles.
And remember. Just because it says ultra lite. Does not mean it is. You have to check the weights on the yellow sticker.