Do you get the open neutral everywhere you go or just in one location where you plug in? Everywhere would indicate a shore cable problem, only in one location would be your supply receptacle.
As others have said it is not uncommon to have a loose or arcing neutral wire. I have a Ting device at home which is very for detecting power quality issues including open neutrals (which can also occur at the utility providers transformer). I haven't used a Ting in a motorhome. See
https://www.tingfire.com/
My homeowner's insurance (State Farm) got me the Ting for free as a fire safety device. It measures the voltage at 30MHz to detect problems. For intermittent problems it is good to have a recording device which the Ting approximates (you can't actually view the recording but it will tell you the results).
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