I guess if you’ve already got an additional fuse in the extension lead you’re modifying then an additional circuit breaker is not required 😉
I think all of the shop bought leads I saw had circuit breakers, as does every Caravan and Campervan.
To be honest most of the camp site electric hook up’s i’ve used look a bit old and tired. some attached to a gnarly bit of wood, the plastic looking a bit faded and crazed, the small plastic lid covering the RCD switch cracked and flapping in the wind. I’m sure they’re required to be tested routinely by the camp site, but i’d rather have an additional level of protection that I’m personally in control of and sure of, especially when things can get wet.