That MIGHT be normal.
My 2000 3.0 4wd 5 speed w/ 4.10s would run 80 with ease in 5th, down shifting to 4th for big hills/mountains to maintain 80.
I suspect you have 3.73 gears and oversize tires. Mine came with 245/75/16 and I downsized to 235/75/15 and it was like a hot rod in comparison. I could bounce it off the speed limiter (92 mph) in 4th or 5th pretty much at will with either set of tires. It simply took longer to get up to speed with the OEM tire size.
It also consistently gave me 340 miles per tank (19 gal) in mixed driving. The best I could get was 21-22 mpg, with 18-19 pretty normal. I revved the piss out of it some of the time, 2nd gear to about 50 mph, 3rd was good to past just past 70. I hit the rev limiter quite a few times.
At slower speeds, you may spend more time in 4th, depending on the hills where you live. I did have to go all the way down to 3rd to pull some of the MD/WV/PA mountains at 65-70 mph (Interstates).
The 3.0 runs happiest right at or above 3000 rpms and still delivers 18-19 mpg (at least mine always did). Hell, I towed cars with a tow dolly and still got 17 mpg (again, on interstates), but 5th gear was useless unless downhill with a tailwind.
I'm guessing VT is pretty hilly. Keep it around 3000 rpm and enjoy the sound and proper performance. I always down shifted as soon as I noticed the speed dropping. My cruise would quit when it dropped 8 mph from the setting.
If you can't maintain 55 mph in 5th and it's not a fair sized hill, something may be wrong. I'd put the OEM intake tube back on and see if that is your power thief. Wouldn't be the first time I've heard of people losing power with one of those.
O2 sensors could be getting lazy too, if they are still original. Don't bother changing the 3rd one after the cat.
I've also heard if you upsize the exhaust pipe over OEM (2"?), you'll lose torque, especially at lower rpms. My 2 cents with a similar truck, drove it over 100k miles, myself. Now my boy has it.