If the load is a small one, an inverter from 12V is the best way to go. In fact it's the only practical solution for a varying engine RPM source.
For a larger load, you need a separate generator unit with a fixed RPM.
That said, you can still buy 120V alternators that are belt-driven intended for under-hood generation. They were popular some decades ago on utility trucks as an alternative to a separate gasoline generator. Unfortunately they require a fixed RPM from the engine, so they aren't suitable for an RV in motion.
This company sells them: https://www.fabcopower.com/generat/bgen.htm