I believe you plug into the un-marked one.
You should NOT have the on/off button (that's an antenna booster and does some magic between directing the cable to TV to/from the antenna vs. the wall-mounted cable connection).
You should have the TV's input set to cable (vs. over-the-air/antenna).
You should need to re-scan for channels.
OR- at least that's what I've gleaned from the forum this year. I've never hooked cable up to my camper.