That is the way a lot of the vintage RVs are. But these were just 15 amp. That way the critters can't get in. But the older RVs weren't equipped with all the mods we use today, so we need 30 or 50 amps. Then there is going to be someone who uses too lightweight a cord, and starts a fire. I think that is why they don't come that way, any more.
There's nothing wrong with a 15 amp connector and cord for loads up to 15 amps (like the setup James showed), or with a detachable cord for a 30 amp load the corresponding connectors are used (like the one in Roger's picture): as long as the circuit breaker (or fuse) suits the load, it was okay, and would still be okay.
The problem would be if someone connected 30 amps of load with one of those 15 amp connectors. I think new RVs no longer use the 15 A setup only because most are built to accommodate higher loads.
I have a stock 30 amp setup in my Boler
, but the only time we would even get near 15 amps would be when using an electric space heater.