electric generator quiet at 24/7 level
A combination of External Combustion Engine (Stirling) with water heater could serve small RVs (not just B) well. I think Booster mentioned it earlier. It certainly can be made very quiet. Higher efficiency than ICE engines, biggest problem would be that this technology is older than the ICE one and the new King of LPG Fuel Cell is almost, almost but not yet here, perhaps tomorrow or by a Friday (quote me in 5 years). Low temperature Stirling combustion wouldn’t produce NOx and with open flame combustion no CO as well. We just got a small Stirling model for our grandkids to explain physics of energy transformation. See video below.
El.Ma. Electronic Machining s.r.l. - 500W - 1500W Cogenerative Stirling engine
Another possible combination of quiet electric generator with water heater could be based on Peltier elements. Not as efficient as potentially 50% Stirling but a solid state simplicity.
Limiting a small generator market (not for direct AC) to B class only is likely way too narrow, it is most likely a 13'-30’ RV market.