John - If you notice on my floor plan you will see my 1500WATT PSW Inverter and it connects to the outside wall right next to the 30AMP Shore Power Cable. When i want to connect my whole trailer to the 1500WATT PSW Inverter I just connect up an adapter cable.
This solves the possibility of not having two 120VAC sources connected up at the same time. I become the human change-over relay...
With just 1500WATTS available I have to shut down alot of things as well by flipping circuit breakers.
If you hardwire in parallel to the shore power cable with a cable that will plug into your planned inverter then this MALE PLUG will be hot when you are connected to SHORE POWER CABLE.
This is a HUGH NO-NO...
This is my converter setup and the flip lid outside connector on the right is what the 1500WATT PSW Inverter is hard wired to. It is perfectly safe using the RV30A-15AMP adapter. NO BARE HOT pins you can touch...
This setup will do what you want to do without using switching relays... It also keeps you from having to tie into your 120VAC wiring also.
the only possible problem will be your INVERTER may trip your GFCI receptacles if you purchase the wrong model of INVERTER...
Got to think being safe... It won't be you that gets hurt - it will be your dog or one of your kids or wife or the repairman that is working on something for you etc...
Roy Ken