Paul, I'm not sure what year your 3121 is. Mine is a 2019 3791RD and, like Mike's, the only thing my inverter runs is the refrigerator. I think I remember reading that the 2021 model added more inverted outlets and more solar. I'd start by tracing from the inverter to the auto transfer switch and beyond. A circuit sounder would be very handy if you can get your hands on one. There are several on Amazon relatively cheap...
https://www.amazon.com/s?k=wire+trac..._ts-doa-p_3_11