Need help re. Jeep catalytic converter...

4.0s are not known for great mileage. My '01 Cherokee averaged about 16 mpg on a good day.

The cat is likely not the problem. You would have a MIL on and if it was in really bad shape, it would be flashing. If there currently is no MIL on, then there is no pressing issue with any emissions-related equipment. You might want to have someone check you fuel usage map with a scan tool though.

O2 sensors do not produce codes by themselves unless it's a heater problem. Codes usually appear when there is extra air in the system from an exhaust leak or intake gasket creating a lean condition. The computer compensates for the lean condition by - you guessed it - dumping more fuel.

Low mileage could be do to any number of things including tires, driveline/trans issues, worn out engine etc. Check for an exhaust or intake leak. Clean the throttle body and run some fuel system cleaner through it, see if that helps any.