My first camper build I had an inverter that powered the outlets. It was isolated from the roof Air cond and from the propane fridge. It worked on switches that allowed gen, shore or inverter power.
For the inverter part I mounted the inverter under the bench, plugged a plug into it (made my own using electric wire). I ran the hot wire through a toggle A/B switch that would toggle between the circuit breaker panel or the inverter. (The circuit breaker panel had set-up to toggle safely between shore and generator). So, if there were another source of power plugged in (gen or shore) the outlet would still only work with whatever was switched on, not what was plugged in necessarily.
The outlets actually went through a power strip that could trip, so that added a little more safety.
IDK if this was up to code or how safe or not safe it was. The hot wire was isolated, not the ground or neutral, but it worked for a very long time with no issues.
Using extension cord-type plugs was fine and safe for what I did. There was no need to "hard wire" anything to the inverter.
The isolated items were isolated from the inverter because they were powered from the circuit breaker, which was not powered up when the inverter was on because of using the toggle switch that disconnected the link between the outlets and the circuit breaker (and connected the outlets to the inverter when on the B side of the toggle - the A side being the breaker box).
The circuit breaker would in turn send power to the toggle switch witch sends power to the outlets that you would want powered by the inverter.
In my case the air cond outlet (actually hardwired) and the fridge outlet did not ever get inverter power bc they needed too much power and when not on shore power I use propane for the fridge and use a generator for the air cond. It was possible to use shore power or gen power for air cond and fridge while at the same time using inverter power for the outlets. Just flipping the toggle from A to B isolated the hot wire from inverter or breaker box.
Of course the breaker box ground and neutral wires were wired directly to the outlets and to the inverter ground and neutral, so all that was connected all the time.
Thats a long answer to say, yea, just use your extension cord idea.
Now that I think about it, I probably shoulda not have used a toggle switch, but rather a relay switch as the toggle could have become hot, or might have been always hot. I think it was designed to not conduct electricity from the wires to the toggle, but a relay would def not do this.
