2000 Ford Ranger reduced gas mileage

At 118,000 I noticed a drop in gas mileage; I'm averaging 15 mpg, down from 18-20 mpg. I had the computer scanned at the Ford dealer, no problems; the tires, pcv valve, oil, etc; has all been changed. It seems like the rpms are higher now than what they used to be, for instance, at 65 mph the rpms are 2600; I thought they used to be around 1800. Transmission seems to shift, manually and automatically okay, I just can't explain the drop in gas mileage. The only changes I've made is having the heater on, all the time, and I took the tonneau cover off. That shouldn't account for a mpg drop should it?