Converting Onan from LP to gasoline

I also have a Onan 5500 LP and I agree it uses lots of propane. As mentioned above propane burns much cleaner than gas. Propane gensets tend to be more reliable than gas because you do not have to deal with carb or fuel issues that plague built in gas gensets. Have you considered simply carry more propane vs. converting to gas. My rig came factory equipped with 80 lb propane capacity.
A more viable solution maybe to plumb one or two additional propane tanks to your genset.