Hi,
Full output may be available but the alternator duty cycle won't allow you to do that for long.
I can safely run my fridge which is 325 watts. If I run the 1200 watt water heater in about 10 minutes the voltage starts to sag from the chassis. I've done this at highway speeds.
I have observed over 70 amps going to the "house" when doing the above. I have doubled up on the charging path and use automatic circuit breakers rather than fuses.
If the inverter is large enough it will draw from the house battery.
Unless you "beef up" the charging path you probably won't see more than about 10 amps coming from the alternator. That may be enough to keep the inverter from failing due to low voltage.
