2wd Silverado with 5.3 liter V8 Automatic or stick.
With any luck you will pull 20 or better on hwy. I've heard better reports from customers that bought 2wd than 4wd but overall the 5.3 is a pretty good motor for gas mileage.
A 4.3 Blazer, and 4.0 Explorer and a 3.9 Durango won't do better than 20mpg unless dropped from a plane.
I'm not a good source for mileage on bigger jap suv's and trucks, I don't deal in them, they just don't sell well in my area.
Rav4, Escape, CRV, Sportage(UGHH!), Vue, Element, xB, are all maybe on your mind cause of the mileage, but none should be used to pull a 3000 lb boat especially out of boat landings, 1500 lbs should be considered Absolute Max for these vehicles.
Car business. After reading my post, I guess I assumed you are talking about a used car, not new, I'm a used car dealer, so I forget all about new, sorry.
Good place to go: http://www.fueleconomy.gov/feg/sbs.htm
Gives side by side comparisons new and used, I send customers there all the time.