Typical install is some 20amp wire in wire-mold along the ceiling to the roof side.
Then tucked behind trim/curtains running down the side to an installed port that exposes a 20 amp plug on the outside of the camper with a rubber plug/seal. (Picture is 15amp but looks mostly like that)
As described above there is a corresponding port in the bottom half of the unit under where the roof port is that houses a female plug end and cord that is long enough to reach from the bottom to top side port when the roof is raised.