Yes replace the fuel filter any time that you have lack of power.
You can get one bad tank of fuel, and it might take 2 filters to correct the problem. It is best to learn how to change them yourself, if you can do that sort of thing yourself, and keep one in the truck at all times. Even if you can not change it, you might be camping in someplace like Yellowstone, where the nearest Napa store is 75 miles away, but someone can change it locally.
If you did not have on in the truck, it would mean parking the trailer (to no damage the engine while it is starved for fuel and pulling a load) and go get one. . . Back in the 90's when I had my truck, it was more likely to get a batch of bad fuel, when steel underground tanks where more common. Back then, it was not ab-normal to change filters every 4 months, while some would last 2 years.
As for the fuel mileage increasing, the less power that you are making, the better your mileage should be. Try driving it like there where a egg between your foot and the right pedal, and slow to 35 on the hills, you will see tremendous increases in fuel efficiency and MPG figures too!
Good luck,
Fred.
