Generator conversion

I would stick with gas.

Onan does / did make a dual fuel carburetor for some of their RV units, I am not sure about the KY 4000 though I do know they also sell the same generator as the LP3600 (propane has less energy in it than gasoline)

I converted skid steers and fork lifts to run on both and many times they would come back because their job was over and when they put it back on gas the carb was gamed up. If you switch it back and forth often enough it will work but I would pick one or the other not both.

I would suggest straight gas or propane, I have had a ton of grief with dual fuel systems. The propane dried out the carburetor gaskets causing them to leak profusely when switched to gasoline.
Also keep in mind that power and fuel economy will drop about 10%. It's a tradeoff for the convenience.