Most houses have multiple 15A rated plugs on a 20A circuit. So in addition to the RV loads house loads may also be present.
A 20A plug on a dedicated 20A CB is not as common in a house. But voltage and wiring to the plug under load are factors for both 15A and 20A plugs.
The salesman's advice is sound and remember guys he has no idea what other loads in the RV or 20A ciruit will be present or the condition of the plug. For smaller A/Cs or on a case by case basis using a 20A plug may be OK.
One of my A/Cs on my last 2 rigs draws 15.5A and no way would I ever consider running it on a 20A plug. In fact I don't run the 13K BTU A/Cs on 20A.