1. Regular. That's what the 4.2 is spec'd for.
2. Probably better with it on. That's what it was designed for. You can google up the physics behind that.
3. Depends on weight. My grandpops had a homemade tonneau on his (RIP) 2000 SCab and I don't believe it hurt MPGs at all.
4. Stomping on the gas occasionally. I don't go overboard to hammer the pedal, but I am aware of the peak torque and where it shows up. In other words, I don't granny it by going up a freeway on ramp at 1400 RPMs (big fuel waste) when the peak torque kicks in at 3500 RPMs or thereabouts and the engine really doesn't start to pull until about 1800 RPMs or so. Use what you've got! Get up to speed quick then cruise.

I see too many posts here and elsewhere that read much like "I shift at 1500 RPMs max no matter what, but I get 10 MPGs with my 4.2! HALP!" See the problem? I get ~19 MPGs mixed. I made a roadie to Florida a few years back and got about 19.3 doing 80 on I-10. Doing 55 doesn't improve that noticeably unless everyone is following a state trooper and is afraid to pass and I can stick the tach at ~1400 RPMs, because a two-gear kickdown to pass anyone is going to drink some gas. (I did the same trip last fall in the same basic manner in my SCrew and got 18.8.)

Gotta' know that engine powerband.

 

It's gas with 10% ethanol. In other words, the crap you will find in 90% of the gas stations these days.

 

1. Regular.
2 and 3. No clue. I have a soft tonneau and have since I bought the truck.
4. Keep the highway speeds down. This has the biggest effect of anything, other than the obvious maintenance items. At 55 I get 23. At 75 I get 17. At 85 I get 12. Try to keep it under 2500 rpm, that's when the IMRC's open up and that drinks gas. Also, on the highway a headwind/tailwind makes a noticeable difference - as much as a 4 mpg difference at the same speed. Around town you are going to get around 15 to 17 no matter what you do.

 

If you want good gas mileage, don't buy a full size truck. None of those things really matter that much and will not offer a notaicable gain in fuel econemy.