Some terminlogy
220 and 120 do not really apply in the RV world.> So please do not describe plugs and outlets that way
50/30/20/15 amps, that is how the plugs and outlets are described.. YES a 50 is 120/240 volt (not 240 but 120/240 or 120/240/120) the others are NORMALLY 120 volt but I've seen a common 20 amp outlet that was 240, and so labeled.. Used it in fact.
Extension cords are often light weight cords, not up to a full 15 amps, let alonw 20 or more
30 amp plugs and 50/15 adapters are also rather close to limit.. Now I've never melted a 50 amp plug, but then this RV draws. at most 50 amps.. LESS than 30 per leg .
The big thing with plugs and sockets is tarnish or dirt.. Nice bright copper to nice bright copper = a good connection, low resistance.
Black = High Resistance, poor connection, Melted plastic
ONE: keep plugs CLEAN, polish if needed (Emery cloth works or a wire brush for plugs.. Sockets are a bit tricker and I do not recommend you try to clean them less you already know how)
Keep spares handy for 15/20/30 amp
Now what is the difference between 15 and 20 amp plugs?
15 amp has two parallel blades. IF one is "Wider" that is the neutral
20 has two perpenduclar blades kind of a "T" shape.. The top of the "T" is the hot lead and the upright (As shown) is the neutral
The outlet will normally have a 'T" shaped neutral slot that way it can take either a 15 or a 20 amp plug.. I have both kinds of plugs in my RV. NOTE that20 amp plugs will not fit the standard RV "Quick box" type outelt.. and for very very very good reason.. they are only rated for around 10 amps peak.