Any circuit fused for 30 amps must use a minimum of 10 gauge copper or 8 gauge aluminum.
6 gauge is required for 50 amp. Consider you are making a dog-bone, starting with 30 amps and passing on 30 amps passed the dog bog, I see no need to throw a 6 gauge in there, which is what is needed for 50 amp. But, nothing says you could not use anything heavier than 10 gauge? Just don't use anything less.
(If you were going from 50 to 30, then I would strongly advise making the dog-bone with the 6 gauge.)
Edit:
I'm plugged into a 30 amp box right now and my camper is 50 amp. The dog-bone is store bought. Under this circumstance, basically no matter what I do in the camper will never trip the main 50 amp breaker in the camper. Once the power usage reaches 30 amps, it will pop the breaker at the shore power box. So, the maximum amp draw from the breaker box is going to be ONLY 30 amps, never anything more. So all the wiring needs to be at least 30 amp rated. (Hope this makes sense).
