A fuel injected V6 (1987 model year, like Keith's) should get about 18 city/23 hwy. New small V8 GM trucks get about 15/18.
All of the big 3 trucks for 2014 are supposed to get around 23-25 highway - with cylinder deactivation, turbo boost, shut off at idle, etc., depending on whether it is a GM, Dodge, or Ford. Of course, more modern trucks bring their own set of problems with computerized controls, injectors, etc., not to mention the big dollars spent (lost?) on the purchase.
Unless you intend to drive your truck a lot as a daily driver, any improvement in gas mileage won't make up for the additional dollars spent on a newer or new truck. I have bought lots of new GM trucks and SUVs in the past 10 years, and have also had one or two old squares at the same time. If I had only an old square as a daily driver I would just get a new truck instead - these old trucks are full of repair surprises, especially when you work them hard. I have always kept an old square around to use as a truck because I never wanted to get the new truck dirty.
From a financial point of view, most people would be better off renting a pickup for the few occasions when they really need one.