ATS switches select between two 120 volt sources, and send one or the other of them into the RV. If it's built into an inverter, it switches between an external 120 volt source and the inverter and sends one or the other into the RV.
The reason you see more inverter based transfer switches is in commercial use, electronic equipment is usually fed the cleaner power from the inverter and battery while a charger keeps the battery topped up. This way momentary surges and interruptions in the power won't affect the feed to the equipment. If the UPS fails, the transfer switch places the equipment directly on utility power so it keeps operating until the UPS can be repaired.
In your case, you want the ATS to select either shore power or the generator, and send one or the into the RV.
Here's one example of a Line-Generator ATS, you can find more by doing a Google search using that term:
Parallax Power ATS 301 Line Generator Switch
The confusion between a 30 amp ATS and a 50 amp converter is there's a 10:1 ratio between amps at 12 volts vs. amps at 120 volts. A 12 volt, 50 amp converter will only draw up to 5 amps from a 120 volt line, plus a little extra for conversion losses.
A 30 amp ATS refers to the 30 amp shore power cord and plug feeding the RV. It has 3 conductors, one each hot, neutral and ground.
50 amp RVs have a 4 conductor cord and plug. They have two hot leads, neutral and ground. The two hot leads are each 50 amp circuits so you can draw up to 100 amps at 120 volts from the pedestal.
