I want to install a 50 amp breaker in my cabin can I add a 30amp and a 20 amp breaker to handle all my appliances and lights, will the 50 amp breaker protect my rv?

You do not add two or more breakers to get the total amps you want. If you could you could use three 20 amp breakers to get 60 amps, THIS IS NOT how you do this. Each breaker is designed to carry a set number of amps. If you need 50 amps, you need one double 50 amp breaker.

If your RV is wired up with #6 or better and rated to supply at least 50 amps, then then a 50 amp breaker will protect it. The breaker feeding a panel does not have to equal the sum of the breakers in the panel. You seldom have every breaker drawing its max.