If your coach has separate brake and signal bulbs and the towed doesn't then you need a convertor,on the coach, to bring the lights through on 4 wires.
Left signal/brake , R signal/brake , park and ground.
If both have separate signal and brake bulbs, then you need to go up to a 5 wire system.
L signal, R signal, brake, park, and ground.
If you have common signal/brake on the coach, and separate on the towed, you need a convertor on the towed and diodes to keep the power apart.
Wiring diagrams will come with the convertor.
If your picking up your power from the factory coach plug, and the coach has separate signal/ brake it may already have a convertor mounted on the coach.
99DSDP 3884, Freightliner, XC, CAT 3126B, 300 HP /ALLISON 3060
2000 Caravan toad, Remco & Blue Ox.