Poor fuel economy

The troubleshooting information I 'm going to present here are general information that can be usesd to assist in determing problems with most makes and models of vehicles, However, , for the proper spesifications and more detailed troubleshooting information for a particular vehicle, refer to vehicle service manual;
However, I doubt the oil change has caused this problem.

Start checking for a binding chocke valve or linkage, Too rich idle mixture, power valve stuck open, restricted exhaust system, Dragging brakes or underinflated tires, Defective transmissin or drive train, Misaligned wheels
this is just to name few.

Thats a hefty loss in a short period. Something that bad should leave clues, such as smell of raw fuel, excessive drag or a performance change.

I would recheck my math first. Then run another half a tank and recheck the mileage.

If your thermostat is stuck open, You will take a hit on the economy. The more short trips the worse it will be overall. How long does it take before you get hot air from the heater? The motor wants to hog all the hot coolant until it gets warmed up, then it can back off on the fuel consumption. When it's warm it will share the heat with you. If the stat is stuck open, there is more volume of coolant to heat up, causing more fuel to be expended.

Another thing that may affect it is if the computer "thinks" the engine isn't getting hot because of an input, such as an coolant temperature sensor.

I am on the verge of saying more than I know. In our shop we can monitor both the wave pattern of the sensor and the actual temperature with our scanner. I don't want to send you on an expensive wild goose chase.

Other inputs that I would be more apt to consider (since the truck heats up promptly) would be the O2 sensors. If they both weaken over time, the CEL(check engine light) won't necessarily lite up. They are compared to each other and if the comparison falls within an acceptable range, they are assumed okay. When one finally falls under it's specific parameter it will trigger the CEL. You replace it, now the CEL comes on becasue the new good one is compared to the weak one because the range is so far apart.

Mathiaso has mentioned tire inflation, I assume that was checked. Air restriction such as a plugged air filter may be part of it. Since it happened so sudden, I'm inclined to think those are fine.

You mentioned it runs a little sluggish. For the heck of it if you can find a scanner (autozone?) See if any codes are present. Codes can be set withour the CEL illuminating. Without checking the O2s, I don't know they are bad, but may be : How old are the spark plugs?

They are the double platinum plugs, but they don't always get 100,000 miles, some go more some go 60,000. Post what you find out from the garage.