Newby has a wiring question

Hello fellow truck camper owners. Here is what I have 2000 Ford F250 with a Sun-Lite pop up camper All the camper power is from the trailer Plug



the camper power comes from the left front and has this plug. I am going to be pulling a small box trailer with a flat 5 power plug
the whole setup seems a bit crude how should this be wired? a plug inside the box?