15/20 amp input

My Inverter is a Multiplus II 12/3000. I have a generator that has a 20 amp plug. I’d like to be able to plug my Multiplus II into that generator or my 120 plug on my tow vehicle. Is this possible. No matter how I try to configure the Multiplus, when I plug anything less that 30 amps into the inverter, I show no incoming grid power and no charging happening. Is this even advised?