Rhust, I also power my FWC from the fifth-wheel power connection in the bed of my truck, a 2011 F-250. I like it.
You will need to decide whether to keep or remove the plug that will come on the camper's umbilical cord and normally plugs into a FWC-installed receptacle in your truck bed. That's the Attwood or MinKota or Marinco plug and receptacle that others were talking about.
If you keep that plug on the umbilical cord, you'll need to fabricate a simple three-wire jumper cable with that style connector on one end and a seven-pin connector (using only 3 pins) on the other to plug into the receptacle on your truck. Your alternative is to cut off the umbilical cord plug and wire it directly into the seven-pin plug.
Be aware that either way you may need extra-length of wire because you'll probably have to plug and unplug from the bedside receptacle before you actually slide the camper into the bed of your truck. Maybe some people are able to plug/unplug through a turnbuckle access in the camper, but I'm not that flexible.
The three wires from the camper are ground, running lights, and a connection between the truck's charging system and your camper battery. In many trucks, charging power is automatically disconnected from the truck's charging system to the battery when the truck's ignition is off. If so, you won't need the auto battery disconnect that FWC installs in the camper to keep the camper from running down the truck battery overnight.
Also be aware that given the length and smallish size of the wires from the truck's charging system to the camper battery and back, the charging system won't be able to anywhere near keep the camper battery topped off or provide enough current to keep a 3-way refrigerator on DC from draining your camper battery. But using the fifth-wheel connection is still a good idea when available in my experience and opinion.
Ed
