Dirty Black Rubber Caps

I used Red Max Pro/Zep on my 96 Southwind. I also used it on the windshield rubber seals. It keeps them from streaking.
Or, try Protectant 303. It may help keep them looking new.

I assume your connectors on the trailer are the male type. Get a female connector and fill the cable end with a sealant. Now you can screw the connector onto the trailer connector to protect it.

I think the OP is talking about the RUBBER CAPS that cover the connections on the side of the trailer. I just use some tile-x on a paper towel and wet the caps and let them soak for a couple minutes, wipe off and it makes them look like new. They do seem to be a magnet for dirt and mildew.