Does the buzzing stop if all loads are shut off in the RV, or if the cord to the house is disconnected?
If disconnecting the cord stops it, then perhaps there's some current flowing between the neutral and the EGC because they are bonded together in the RV as well as in the house panel. Not having a good N-to-EGC bond in the panel would aggravate this. Now why this would cause buzzing is questionable, but perhaps there is some light gauge magnetic material like steel between the N and EGC conductors that is buzzing due to the magnetic field produced between them.