First action is a power audit.
You will need a Corded light, a volt meter or one of those outlet testers with the 3 lamps and gfi test button along with a fine point sharpie, a roll of tape and a note pad.
Make a sketch of your rv and note where all outlets are located.
Turn all breakers OFF now look for anything lit up or operating and verify why it is on if 120 volt or note if 12 volts.
Now turn on ONE breaker and note the breaker number.
Check every outlet with the tester and check all of the devices and note on your drawing which breaker is now on next to the device as well as ] lacing a piece of tape on the device and marking the breaker number on it.
Turn that breaker off and turn on the next one...
Now that you have the ac power mapped out you will know if the outlet for the fridge is dedicated or shared.
If shared then shared with what?
If shared with a light or unused outlet then fine to intercept at the breaker panel.
If not then you will need to pull a new circuit.
If in conduit do NOT use Romex as it is not needed.
You can use direct bury and run along frame rails if needed.
For inverter look for one with built in transfer switch and minimal overhead or simply buy a relay with a 120 vac coil and make one yourself as the relay is maybe 30 bucks or less.
Routing the cable can be difficult but frame rails or basements or under cabinets are all options.
Tony & Lori
1989 Country Coach Savannah SE