You might consider building your own one branch circuit 20A max transfer switch for $ 20 in parts using a standard electrical box with cover and romex clamps using
Two SPDT Power Relays. It's a pretty simple circuit given your post # 1 I'm sure its within your capabilities to wire it up. One relay for hot and one relay for neutral. NC contact is branch circuit power feed. NO contact is inverter power feed. Coils to inverter power feed. Common contacts to load.
Then you can use any replacement inverter with a remote on/off switch in place of the $ 400 Xantrex with integrated transfer switch.
Your only limitation will be the current 150 amp DC fuse and 2 AWG wire that Winnebago uses to feed 1,000 watt Xantrex Inverters, that will continue to limit the inverter size to 1,500 watts max. You will want to do a Neutral to Ground bond connection on the inverter side of the transfer switch relay contacts so that any GFCI protection in devices on the load side of the transfer switch will work right.