12 volt stuff only works with generator sometimes

Your house batteries are apparently not being charged, or it would be seamlessly to transfer from shore/generator power to battery power. Check your house battery voltage, if anything less that 12 volts place a battery charger on them to charge them at least overnight. Voltage should come up to 12.7 volts when charged, and hold a charge for several days, if they are good. Both shore power and the generator will power the converter, which is suppose to supply 12 volts to the batteries and house loads.