Each Host has their own rules about generators. Most Hosts will have this information on their profile, but you are always able to ask the Host about generator usage when you request a stay with them.
Most of our Hosts are fine with running generators sparingly though. You will also need to consider if they are allowed by local regulations or ordinances. If it is quiet, you should be okay. You will need to be respectful of other RV's or neighbors who are near to where you are parked.