1)why is the fuse popping more often with an extension cord? how long can you run an extension cord and what size to prevent this?
The extension cord adds two things. Additional resistance, = more voltage drop this can cause the A/C to take just a fraction longer to start and trip goes the breaker. IT ALSO ADDS A VERY SMALL LOAD yes. a cord even with nothing plugged in presents a very tiny load to the generator.
2)do I need to up size my generator to prevent further overloads? or do I have another issue with the RV?
I think you are good. just need earplugs for that generator.. Since it's not trippign the breaker when you don't extend the cord you should be good.
One other thing are you using a 30 amp extension cord or a 16 Ga Outdoor "15 amp".. Upping to a 12 ga cord may help or better yet a 30 amp extension cord. I have one of those cause from time to time.. I need it.