Generator 30 AMP Outlet vs 20 AMP extension cord

You can use your cord but you should restrict it to loads that are 20amp or less.

No, the 20 amp rated cord does not have the same configuration as the the 30 amp outlet on the generator. It will not physically fit. You can use the adapter, but keep in mind that the generator's breaker protection is for 30 amps, but the wire of the cord is only rated for 20. Therefore, under heavy load, your cord will get hot, and ultimately may heat stress the conductors, further damaging them. Just play it safe and buy a heavier gauge cord. I know they can be expensive, but if you order them online, from sites like Amazon, they can be much cheaper.

they are readily available at any RV center since most RVs use 30 A hookups

Yes. They are . I was merely pointing out that pre-made generator cords can be rather expensive, in fact, exponentially expensive for longer distances. I was just pointing out a more cost effective source.