If the engine has high carbon valve seats it can suffer damage to the seats, and rotory engines ie Wankel require fuel lubrication of the rotor tips, and suffer excessive wear of the tips.

As several generator companies offer LPG as an alternative fuel option, and CAT do an engine that will run on diesel, petrol, or various gasses by selecting the choice of fuel from the panel, but they are 12 ltr V12 not used in many RVs.

Generator manufacturers don't normaly go for high performance engines, 900, 1500, or 3000 depending on the number of poles, sorry in the US those RPM figures will be corespondingly higher.  I would have no hesitation in running a small domestic generator on LPG, add to the fact the exhaust gasses are mainly CO2 and water, the Nox products are minimal.

Hope this helps