My 1991 F-150 has a 4-flat wire pin connector to connect to a trailer. It is connected to the OEM tow wire harness through a T-Connector. The 4 wires connecting the "T" to the 4-flat pin connector are green, yellow, black and white. I know the white is ground, the green is right turn, and yellow is left turn......what is the black for? Is it for the tail lights?

The second part of the question is....I have an older slide in truck camper. I am trying to wire it to my truck to run power to the running lights on the camper and charge the battery/power accessories in camper while the truck is running. How do I use the existing connector(see above), what wires on the truck OEM harness do I need to tap into in order to run the accessories and the running lights?

 

That wire is brown not black and it is the marker/running/park lights.

That brown wire will give you power for your running/marker lights in the camper. Most people install a plug in the bed to plug the camper in. You can also find this wire along the frame rail. It is brown with a white trace so you don't have to run another wire from the rear of the truck. Since you only have a 4 pin trailer connector you probably don't have the tow package in your truck and the relays, wiring and fuses are not there for the trailer auxillery tow/charge. You will need to run a fused or circuit breaker line from the battery to the camper. Since this will be a HOT line make sure and fuse it next to the battery and protect the wire with flexible conduit or a heavy sheathed insulated wire. I run 14ga 2 strand trailer brake wire that is heavy sheathed and I mount the ground dirrectly to the battery as well as to the chassis or frame.

Depending on the camper and the amount of lights you are trying to run you might also want to consider installing a relay to power your running/marker lights. The truck light switch is commonly blown out or melted from running to much of a draw in the pre-1997 trucks.

 

I have one more questionn ow....since I will be running a new power line to the camper from the battery, should I install an isolator to prevent my truck battery from running down, and have it connect directly to the alternator?

 

Without an isolator the truck and camper battery will have the same charge. You are effectively doubling your battery capacity of the camper. In my experience isolators fail more than they function. You won't find out that the isolator failed until you need the camper battery. At that point you can easily wire around the islolator to charge you camper battery once again.
Long story short I wire dirrectly through a fused connection to the battery, not the alternator and leave the isolator out. I'm sure others will disagree, but again I have just seen most isolators fail after a very brief time.