Slight drop in fuel mileage and acceleration power

I have a 1997 Ford F-150 4WD, 124,000 miles. I am one who monitors fuel mileage each time I fill up. Over the past 3 years I have been struggling with trying to pinpoint exactly what may be the cause of this ongoing delema. My truck used to get 14.5 to 15mpg consistently. I keep my truck tuned up regularly and change fluids and filters at regular intervals. At first, when all this began, I notced a slow drop in mileage that was somewhat erratic, ranging from 14mpg to 11.2mpg. Most of the time I average 13- 13.7mpg now. I have had the catilytic converters replaced 18,000 miles ago, O2 sensors replaced 45,000 miles ago. Also I have replaced the EGR valve, IAC valve, TPS, VCV, MAF and EGR control sensors over the peoriod of time in question. Recently I performed a vacuum test which the results show good under no load conditions(17-18hg at idle and 20-21 at 2500rpm), but under load or drivability shows a large drop in vacuum durring normal acceleration, down to(7- 4hg) then will recover and stabilize when crusing speed is reached, 45mph to(10-12hg). When accelerator is mashed to the floor, there just isn't the get up and go it used to have. The truck has a steady idle and acceleration, no knocking or pinging, jerking or anything else out of the norm. No trouble codes show on the scanner. I still need to do a compresson test and coil pack test, but the spark plugs show normal wear. Can you help me with any suggestions?