Here are a couple ideas. First,
tv and radio reception where you wouldn't get it elsewere
The phone line is a blank line from outside to inside the camper, you can find adapters online from phone jack to RCA or
phone jack to Coax, so there are possabilities. hook up the adapter to your stereo speaker output inside and another adapter for to hook up external speakers. the same for an outside TV. Or make one form an old phone cord to what ever end you want with parts form Radio Shack, there just wires, and with no external source it should be no problem
The phone wires aren't fully insulated, so the clarity won't be digital per say