You are contemplating a relatively simple modification (utilizing the “Prowatt”), which is difficult (for me) to describe in words.
E-mail [or PM, or tel: (807) 622-3600] me a FAX number, and I’ll fax you a simple wiring
You don’t need any Neutral to Ground Switching. The Prowatt could be wired to always feed the (cut & isolated) “Essential” circuits off battery
power without any switching at all ...
or the preferred method, where
The (isolated) “Essential” circuits could be wired to a double pole - double throw (DPDT) switch, wherein they are powered from either
the AC Shore Power Panel, or
This system can always power the "Essential" loads, but
requires that you manually switch the "Essential" loads to Inverter Power (When Shore Power Unavailable).