[RMcNeal]

Paul, not sure what year/model rig you have. My 2019 3791RD also only has one receptacle for the fridge. That receptacle is wired to the auto transfer switch mounted to the ceiling of my basement pass through. From that auto transfer switch, I have 120v feed from the distribution panel and 120v from the inverter, also mounted on the ceiling of the basement pass through. The wire from the ATS to the inverter has a normal plug on the end which is plugged into the 2000 watt inverter about 1 foot away.
When shore power is on, the ATS passes 120v to the fridge receptacle from that source. When no shore power is available, the ATS passes 120v from the inverter, assuming batteries are switched on and have enough juice. The ATS will favor shore power as its source, so when both the inverter and shore power are supplied, shore power is passed to the fridge. That way I can leave the inverter on as a backup power source when the electricity fails.
Just to note so there is no confusion, the ATS is a 120v auto transfer switch specifically for the fridge circuit, not the same as your ATS that comes with the generator prep. That ATS would need to handle 240v/50Amp.