Keep what you have. It's more expensive to replace your Explorer than it is to add gas. You tow about 600 miles a year, so MPG is hardly an issue. Even at 10MPG, that's only $270 or so in gas at current prices. A replacement vehicle will cost far more. Last but not least, I don't know of anything on the market that can do what you're wanting. Get a second vehicle that gets good MPG, like a base model 4-cyl Elantra, Sonata, Fusion, Altima, etc., and keep your Explorer for when you need a tow vehicle or other occasions as mentioned.
I have a 4Runner and an Accord. One averages 15.3 mpg, the other 27+ mpg, and those averages are with approximately 60% city driving because I have a short commute to work. My suggestion is to keep the Explorer and to buy an economy car. Less outlay, better mpg, and you still keep your tow vehicle.