I think that it needs to be a 20 amp, to a 30 amp adapter...
(I'm not sure how to explain this properly...
)
You'll have the 30 amp cord unwound from the rig, and the Male end of it will be dangling out these in the breeze...
When the 20 amp cord is plugged in at the power source: it's male end gets used that that end, and you're left with a 20 amp female end: ready and waiting!
(I need a drink of water...
)
The adapter should be a "Male 20 amp; to a Female 30 amp"
At least that's how I think it goes...