1992 Ford F-150 HELP!

I have a '92 F-150 4wheel drive. I love my truck but I get the worst gas mileage ever. I have to have some compression problems b/c I cannot get full power with my EFI V-8. It is always downshifting to get the power and it sucks the gas right out. I was thinking it may be an exhaust leak at the manifold, but not sure. I changed plugs and that didn't seem to help. Went camping the other day and in Arkansas there are a lot of hills. My buddy thought my truck was a 6 cylinder b/c he didn't suspect that a V-8 would struggle so much. Any suggestions? Anything would be greatly appreciated.