I just bought this truck for work but I am having a problem figuring out this check engine light

The engine runs good, slightly rough idle and I don't think it is getting the best mileage. I also think it is quite low on power. I cleaned the MAF thinking this may be the cause but did not seem to change anything. I cleaned it with an electrical cleaner that does not leave any residue.

Also, once I reset the MIL code, it seems to reappear after about 50 miles but it will be quicker if I'm using my a/c, which cycles too much (probably low on refrigerant)

I'm not one to throw a lot of money at a vehicle with this many miles so hopefully you can help me. I don't like to replace parts until I'm sure they are bad.

The code I'm pulling is P0712 - System too rich (Bank 1)

Here is the info I got from the scanner:

I/M monitors:

Misfire Mon: OK
Fuel Sys Mon: OK
Comp Component: OK
Catalyst Mon: INC
HTD Catalyst: N/A
Evap System Mon: N/A
Sec Air System: N/A
A/C Refrig Mon: OK
Oxygen Sens Mon: INC
Oxygen Sens HTR: INC
EGR System Mon: OK

Freeze Dat

Engine RPM: 1576
Load Value: 78.0%
Coolant Temp: 188f

Short Term Fuel Trim 1: -5.1%
Long Term Fuel Trim 1: -16.1%
Short Term Fuel Trim 2: -5.1%
Long Term Fuel Trim 2: -16.1%
Short Term Fuel Trim 3: 53.9%
Long Term Fuel Trim 3: 47.6%
Short Term Fuel Trim 4: 59.3%

Speed: 32mph
Fuel System 1: CLSD
Fuel System 2: N/A

Any help will be greatly appreciated!!!