cat con guts burned out???

94 GMC AWD Safari Vortec W engine 4.3 I took it for a drive last week I couldnt make it there and back about 300 kms on one tank ouch. I was towing about 2000 lbs the check engine light was comming on and off.....it seemed to run fine.....two years ago I replaced the o2 sensor because it was plugged up with carbon. I thought maybe the Cat converter was plugged so I got it cut out. It was wide open completely burned out same as a straight pipe. This van gets pretty close to spec milage except when I am towing. The mileage is brutal when towing is this normal? What is the most likely cause for the Cat con failer? What are the other two sensors called near the intake? Witch one is more likely to be bad? I might as well guess because the local guy here wants a arm and a leg to read for codes. ... and beside reading for a code will only tell me what I allready know It burns rich to much fuel gone forever.