What year truck and engine?

The O2 sensor will be somewhere on the exhaust, from the manifold to the Cat. 3rd gen('89-95)CA smog equipped 22re's will have two, one on the exhaust manifold and one after the Cat. Lazy O2 sensors will cause a loss in mpg, but wont be the cause of the hard to start issue.

The timing could be off a little bit which could effect the cold start, also the cold start injector could also be going bad. The timing being a little off could also effect the mpg's, but not drop it to 14.

What tranny does it have? Auto's do get significantly worse mileage than their 5-spd counterparts.

What it is the size of tires on the truck? Stock tires are 28's, 31's which are a common replacement are about 11% bigger, which equals a 11% loss in kilos on the odometer.

Have you done a compression check? Stuck/not completely closing valves will cause a loss in power which=loss in mpg. Any white smoke on start-up? A bad/failing head gasket will also cause a loss in power and effect the mpg's

You could have a few little things going on that are adding up to a lot of mpg loss.