[JRTJH]

It's not really a tough question, rather it's difficult to understand what you're trying to do. There are adapter cables available to split a 50 amp into two 30 amp plugs, one to power your RV shore power cable, then you'd need to adapt one of the 30 amp plugs to 20 amps and plug in your extension cord to power the "other stuff"

The difficult part is trying to understand "why" you'd need to do this. Every power post I've encountered that has a 50 amp and a 30 amp plug also has two 20 amp plugs already wired to it. The issue I have is why would you need a special cable that costs upwards of $80 to adapt for an extension cord when there are two 20 amp plugs already available?

Here's a picture of the cable to split 50 amps into two 30 amp service plugs and a picture of a 30/20 amp adapter to use on one of those legs.