This subject is beat to death fairly regularly. I haven't been on the forum that long and I've seen fresh mpg threads a good half dozen times. I would say the search function is your friend, but the reality is that the search function on this site is trash, and even if you do find something worthwhile (on just about any subject) it's mostly just people chiming in with what Joe-Bob said or their personal experience with whatever they decided to do to justify it to themselves. In this particular case, personal experience is called for, but you never get the full story. To have an idea what to expect, it's worth it to know:
Engine
History
Equipment
Crap kept in the back
Driving style
Transmission
Temperature
Fuel quality
Altitude
Topography
Etc.
I have a V6 truck that has been very well cared for with all maintenance records and everything done on time. It's stock with a 5-speed and I shift at the appropriate RPM for the engine, speed, terrain, and load. There is absolutely nothing in the back save the spare (I even keep the rear seats down). I drive my 4Runner like I drove my VW Bus (slow, deliberate, and defensively). It's generally quite cool here (it's the end of August and I just took my sweater off at 2 in the afternoon), I only buy quality fuels (no Murphy swill for my vehicles). I live at about 8000' and work around 6000' (and roll in neutral almost all the way down). It's not at all uncommon for me to get over 23 mpg, and I've only once gone below 20 (I was driving down to Texas and back). I could probably get better if I were to attempt any sort of hypermiling techniques. YMMV
Also, don't do the deck plate or ISR mods, they will not help your case.