You'll need to do more than one tank or part tank to get close to accurate average mileage. Depending on driving habits, terrain, the season and ethanol content of gas you could average about 20 mpg. I've gotten 18 as the worst and 22 as the best with a MT, but average out long term to about 21mpg right now. If the 10mpg is correct then something is really wrong and no amount of your "minor stuff" list will correct it. I'm surprised you haven't got a code if that mileage is correct. I'd try to verify mileage first then figure out how to fix.
^^^^^^^This.
I've found the winter is wreaking havoc on my mileage. I was getting about 14 in town on E85, but it's dropped to about 10-11mpg. Back when I used gasoline (E10 in MN), I could hit about 22mpg easily enough on the highway, and my best was 24MPG on a 2 hour drive, but never got it again

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If your truck is at 100,000 miles, it might be due for a plain old tune up like plugs and wires. I know when I pulled my plugs at around 98,000 (I had the money then so thats when I did it), the gap was so far off somehow my gap tool didn't even register it. So before you start tearing off fans and stuff, just go over it good to see if it needs any simple stuff. If you just got it, you can't say for sure how well its been maintained. It seems to me far too many people jump on here as soon as they get a Ranger and want to tear into it as fast as possible. I waited about two years before touching mine, and only did exhaust.
As far as trouble spots, the cam synchro gets mentioned alot, and while I've replaced my sensor, the shaft looked fine when I replaced it. Otherwise, I'm at 118,000 miles now and I bought it with 78,000 and only major thing I've had to do was carrier bearings in the back axle and an injector. O2 sensors are regular maintenance (when they go) as far as I'm concerned, along with the plugs, wires, flushing the tranny, and replacing balljoints, so I don't consider them anything out of the ordinary on my 11 year old truck. Only hiccups you might have is if you have a FFV 3.0, but IIRC they got rid of the alcohol sensor that my 99 has in later years. Other than that half the people I know with Rangers - up to 6 at one point - have a 3.0 and they seem to like it, and me and one other guy have FFV's but havent really had any FFV-related issues.