1. The Transfer box has nothing to do with a Auto Start system, so replacing it will not be a factor
2. Most Genset Auto Starts, Both Xantrex and Magnum require additional Auto Genset parts to be used in conjunction with the Inverter/Charger if you have one already
3. Onan used to make a standalone Auto Genstart called the Command 30 or close to that name. I do not know if they still offer it or any Auto Genstart system
4. The easiest and cheapest way is to install a Gen Start that uses Battery voltage as the factor in starting the Genset, If you want the AC units to start the Genset, that will get pretty expensive aftermarket with the wiring and updated AC controls that are required for AC to start the Genset.
5. You need to post Year/Brand/Model of RV and what type Inverter/Charger and model of it you have for use to give you more info. Doug