Highway shifting (possible) problem

[Fireman's_KR]

I've owned my 2001 F150 Supercrew King Ranch 4x4 for about three weeks now. I'm not quite sure if this is a problem, so here I am begging for information. At highway speeds (60-70 mph) my tachometer reads approx 1600-1900 rpm's consistantly. Is the rpm's supposed to drop when I let off the throttle (coasting down the highway)? I guess I would just expect either a shift down or something to happen because (in my opinion) coasting should not keep the motor at the same rpm as when I am in the throttle. I can shift to neutral and it will idle, while coasting, at approx 700 rpm's. Do I have a problem here, or am I just ignorant to the way my truck should operate? By the way, I have a 5.4 V8, 3.55 Limited slip rear end on an automatic transmission (if that helps?). Also when I bought it for the first tank I was getting 13 mpg. Now I am getting 10 mpg. Thanks for any input this clueless guy has!

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[Bluegrass]

For disscussion purposes the trans is in lockup during this time so the rpm won't suddenly drop just because you lift the throttle but go down as the truck's speed slows. This is because the engine is in hard coupling to the trans with no converter to slip under those conditions.
There is no down shifting unless the speed drops down under about 50 where reapplication of throttle would cause a down shift to occurr depending on the engine load the PCM sees.
Technically if you watch real close the engine tach if you have one, you may see it move very sleightly with throttle lift because it very quickly releases and reapplies the lock up as a normal function. Even tapping the brake will do the same action. There is a reason why this happens but not for this discussion.