Recently my CEL started to come on here and there, at when it did it the last time, the truck ran real rough. Went and scanned my codes, came up with a few. The guy at the auto parts store said that his ranger did the same thing and it was the Coolant Temperature Sensor and for the codes pulled, it sounds reasonable. I think I found the sensor and am going to be changing it as soon as I get a free minute but my question is a simple one. My MPG have always been around 16, 15 or so in the winter. The truck is fairly well tuned up and runs fine except for this. I know that this sensor is telling the computer that the engine is always running ice cold, therefore running rich at all times.

Could this be the cause of the crappy MPG I have been having all this time(about a year since buying it)?

if it's running rich, sure - that'll kill your mileage in a hurry.