Reason for poor gas milage

I had a 1995 Silverado Z-71 with 350 engine and throttle body. Got about 13 mpg. Changed engine due to oil consumption. Still got around

Now I have a 1995 Seirra 4x4 with 350 throttle body engine and still get 13 mpg.

Some people say that 13 mpg with 4x4 is normal. Then someone told me that the low mpg is because it is a throttle body. They said that GM stopped using throttle bodies because they only produce half the hp of a carbuteror and this is why I get low mpg. I should switch to a carburetor which is a major conversion. I later was told that this is a minor conversion and that the vica versa (carb to throttle body) is the hard conversion.

Now, yesterday I was told that throttle bodies produce higher mpg than a carburetor and that my problem is probably a bad O2 sensor or a kinked wire or a plugged catalytic converter(which I due not have anymore). I was also told that a new computer chip could help a lot or that my current chip could be reprogrammed.

So, whats the truth? I can't live like this much longer. Should I be getting 13 mpg or should it be higher? Does throttle body get better or worst mpg that a carb? Would a conversion be practical and possible? How do I know if the O2 sensor is bad before replacing it? Will a new chip help? How much help?