I have oil coming out of the square hole on top my rear cylinder on my XL1200. Would it be that little square rubber gasket under the rocker box cover?

 

It sounds like a rocker box gasket leak as you suggest. Any one of the gaskets at the rocker box can cause a leak, although it's the bottom gasket that often fails.
To verify exactly which gasket, it might be worth drying it all off and watching the area closely over the next couple of rides.
It's typical to replace all the rocker gaskets when one leaks but depending on your mechanical ability if it turns out to be one of the top rubber gaskets that has failed, it is pretty straight forward to replace as it's just four screws and although it can be a little fiddly to do on the rear due to clearance with the frame, it's not a big drama.

 

Yeah, so I replaced the metal bottom gasket and the top rubber one and still had the leak. But I finally found it; It's coming from the screw hole on the front left screw of the top cover. It's hard to believe oil can come out of that hole with a allen head screw in it. I guess I'll have to find a rubber washer to put in there. Thanks for the replies.

 

There should be a washer type sealing gasket under that screw, your year used to have a paper style one that can become torn on removal.
They've since been updated a few years back to a metal washer with a sealing rubber ring on the internal diameter.
Good job finding the source of the problem.