I think you must already have a generator in the camper?
You want to use the one existing transfer switch to select between shore, gen, and inverter? That can't be done. You would need an additional transfer switch.
You could use an inverter/charger with a built in transfer switch. You would connect your shore cord to it's input and it's output to where the shore cord is connected now. This would eliminate the need to turn your converter off when on the inverter (you would just permanently disconnect the factory converter).
