120V from Genny But Not Shore Power

[Bru20]

I have been reading the posts for a number of years but have never seemed to have a problem that I needed to ask about. My problem - I have a 2018 3811MS with an onboard Onan generator. I have the unit plugged into shore power (30 amp) in my barn and there is no power to the trailer. I have checked the receptacle and there is 30 amps at the receptacle. If I unplug the trailer and start the generator, the trailer is fully powered. It appears to me that there is a problem in the cord from the receptacle to the trailer. The cord is on a power reel and I am wondering if that could be where the problem is. Hopefully somebody has had this type of problem before and can give me some guidance. Thanks