Robbie,
There are too many variables to say exactly why your gas mileage dropped, but here are some things to consider.
Toppers and tonneau covers do not improve gas mileage....in some cases they may even hurt mileage, as well as adding weight.
New tires might have more roll resistance, may be heavier, and may be inflated to a lower pressure. Generally, 33-35 PSI is the optimal pressure for MPG by minimizing roll resistance while still providing ride comfort and maximum tire wear.
If you have not changed the sparkplugs and plug wires, it's past due.
The factors that most often effect gas mileage are your driving habits that you may not realize you are doing, or things that you have no control over.
Traffic, weather, wind, etc are things that can have a big influence on your gas mileage and you may not even be aware of these slight changes and their impact on MPG.
A really big factor is if you have a lot of traffic on the Interstate and you constantly have to change speeds...ie: slow down for slower moving traffic and then accelerate when the traffic clears...That slow-down and speed-up driving really kills mileage on heavy vehicle like the Sport Trac. One strategy that I have tried and it seems to work, is to drive just a wee bit slower than the average flow of traffic. That means you can drive in the right hand lane with your cruise control on and rarely have to slow-down or speed-up, and it's the speed-up part that really kills mileage.
Your new tires might be a slightly different diameter which could be throwing off your speedometer/odometer. If you don't mind spending about $60 and buy an UltraGauge. This gives you an instantaneous MPG read-out and lets you see exactly what kind of driving saves gas and what driving styles waste gas. Just using the UltraGauge can gain you a couple MPG. It also will insure that your speedometer/odometer is properly calibrated for your tires by allowing you to set the UltraGauge speedometer/odometer correction factor so that you are reading a true and accurate speed and MPG.
...Rich