Dodge durango..11mpg..Bad o2 sensors?

I have a 99 durango 5.2L that get's only 11mpg,no matter how it's driven,I noticed today that when I went to p somone,the engine bogged down to almost no power at all,then the check engine light blinked 3 times and stayed on.These are the codes the computer put out...PO300,PO303,PO304 MISFIRE ON #3 AND #4 CYLINDERS..Is it possible that these codes are from bad o2 sensors?How do I test them?How many does this car have? 11 mpg sucks.