I don't think there would be any change of opinions on LP vs diesel. Physics hasn't changed, so LP still has much less energy (BTUs) per gallon than diesel, and small generators still vibrate rather harshly. An LP generator is really just a gasoline generator running on liquid propane instead of gasoline, so it runs at higher RPMs than a inverter-type generator. Most RV diesel gensets are inverter types, so the comparison is not apples to apples.
Yanmar makes a 3700 watt diesel genset that could have been used in place of the 3400 watt Onan microLite. It runs about 20% higher in cost, though, and I suspect that Onan may be making better volume supply deals for RV manufacturers as well. Yanmar is bigger in marine applications, while Onan dominates in RVs. The Yanmar is also a direct generation type (not inverter), so not sure that it would be any quieter either. I don't know why nobody is making a small diesel inverter generator, but I don't find any on the retail market.