my blazer with a 4.3 was taking a quarter of a tank for me to get to work. i put some sea foam to it the other day then drove it to work monday and it took 3/4 of a tank. i could seriously watch the gauge go down. i did nothing to the motor but the sea foam. i took it to auto zone today to have the codes pulled and it said my Oxygen sensors were bad. the guy told me it only had two so i bought them and replaced them only to find out that there were three of them. it is still getting the awful gas milage and has a hell of miss at idle. is that last oxygen sensor still causing this or is it something else.

 

I would get the truck scanned by a qualified shop... Sounds to me like you may have spent money on parts that you didn't need.
The real question is "what is causing the O2 sensors to act up?"

 

had a common thing happen to mine over the weekend, codes po171 and po174 left bank, right bank, fuel to lean or mean or some garbage. was going to start with the o2 sensors, check the vacuum lines for leaks, the fuel pump. while looking through the the book seen that it was just an easy check for the map sensor, so i check it. with the scan tool it is supposed to read 1.2 volts to 1.6 volts, was only reading .4 volts. replaced the map sensor and it fixed my problem. all the issues i was having pointed toward other things including the codes, but after having the problem for awhile it can throw other codes