You'll need a low-pressure electric fuel pump (to replace the one on the Onan you yanked out of the RV), along with an adjustable fuel pressure regulator to reduce the fuel pump's 4-5 PSI output to just a small fraction of a PSI (which is what the pressure coming out of a gravity-fed fuel tank would normally be).
This prevents the generator engine from running rich or flooding out, due to excessive fuel pressure.
For safety's sake, the electric fuel pump will need to be connected so that it shuts off if the generator engine dies for any reason.
Many portable generators have a 12 volt DC "battery charger" output that can be used for this purpose - basically, the battery charger output powers the fuel pump whenever the engine is running.
You'll also need to add a "prime" bypass switch that allows the fuel pump to be manually turned on for a few seconds while starting the generator.
