Marinco makes a conversion kit and sells the 30 amp cordsets. I did the conversion earlier this year. I managed to buy a new cordset and inlet and do the whole job for about $102, not including some RV caulk. I've included a few pics of the job I did. I paid $36 for the inlet and $65 for the cordset on ebay. Then I bought a cutting board at a thrift store made from 1/8" thick HDPE plastic for $1.
If you buy just the conversion kit, it's expensive and they supply some parts you don't need. But, if you are removing your old "mousehole" cover, you need the kit because it has the adapter ring to be able to fit the new inlet. Or as I did, just buy an inlet fitting and make your own adapter ring. I cut the cutting board with a jigsaw. Simple.
The Marinco kit comes with a connector to use your old cord. But if all you want is the adapter ring and inlet because you want to buy a complete new cordset, then it gets exensive.
You could also buy just a Marinco connector and boot if you want to use your old cord. I bought these at Overton's Marine on the internet as it was the cheapest price out there. I also bought a 30 amp connector & boot so I could use our old cord as a spare.
Shop around on the internet. I found Amazon was a little too high when I was looking and ended buying on ebay. I am not very impressed with the fit of the Marinco connector into the inlet. The fit is too sloppy to me and does not have a tight twist & click feel to it like it should. Next time I would take a closer look at Conntek. I see that Camping World has switched from Marinco to Conntek.
You'll note in my photos that I have the new Marinco cord beside the old one. The Marinco one is way more flexible, esp. at cold temps. due to finer strands and different jacket material (STE vs SJOW).
One nice thing I have found about this conversion is that you gain a big storage space where the cord used to push into. You may need to put in a removeable lid though.
Gil & Deb & Dougal the Springer Spaniel