I also have a 2005 5.4 liter FX4 supercab.
Mine idles at 600rpm, was getting 11mpg until I installed the edge tuner and now I get 14mpg if I drive normally without getting into it much.
And yes, if I run cheap gas the fuel economy drops big time. some in here claim to get 18mpg and then some. But I also am running 35" tires with a 6" susp lift.
If I had to guess, sounds like your truck needs a tune up or more if your running stock tires and such.
If you go with new plugs you had better do your research, there is a link in here that has a video showing whats up with the 5.4 liter plugs if you do a search you should find it. I highly recommend watching it, some in here have been charged $1600 and up to have the plugs changed by ford dealers after they break the ceramic on the plugs.
The plugs are broke into 3 pieces and require a special tool to fix if the ceramic breaks and the grounding insert gets stuck in the head. If the tool does not work then you will have to pull the heads completely off to repair them.
Anyhow, hopes this helps you some. Its 3:40 am right now, Im sure you will get some more answers tomorrow after the sun comes up fer a bit.