New Camper "cub" Manual controll Brisk Air

Since you have a Solid wall and ceiling the 120 wire is indeed in the roof. What you need to do is remove the Inside Ceiling AC cover and check your voltage where the 120 feed Romex connects to the AC. Unless the previous owner modified something, the Romex runs direct from the roof AC to the 20 amp breaker in the breaker box. I would suspect that a loose wire nut connection where the romex connects at the roof AC is loose or burnt. This is a common problem when the OEM's fail to get the wire nuts tight. I would also call the previous owner and ask about any extra plug ins and see if he says there is. You did verify that the White and Black wires at the 120 breaker box, there are NO burnt connections? Doug

PS pics would help