I encounter this complaint in my RV park periodically, and invariably
it will be a neutral connection missing, broken, or loose.
The neutral wire is misunderstood, or many have no idea, as to the
importance of this white wire.
With a good meter, start a the RV end of the line checking line/ground (should get approx. 120v), line/neutral (about 120V), neutral to ground. (should get almost nothing, possibly a couple of volts).
If everything doesn't check there, work your way back until you have found all good readings, then repair the problem.