If it is the old style 30a/240v 3-wire outlet, you CANNOT USE IT. However, IF IT IS A 30a/240v 4-wire outlet, YOU CAN use it for your RV ... IF ...you create the correct converter/pigtail. Treat it like a 50a/240v connection, buy a 30a/240v 4-wire dryer pigtail, buy a 50a/240v outlet, wire the outlet to the end of the 4-wire pigtail, TEST THE OUTLET WITH A MULTI-METER for correct wiring, and if all tests correct you can then use the connection (if you don't know what to test for, DON'T mess with this). You will have two 30a power legs for a total of 60a rather than the normal 100a that a standard 50a RV outlet provides, but you would be able to run pretty much all you normally run in a 50a RV. IF YOU RV IS WIRED FOR 30a ONLY, simply plug a 50a to 30a pigtail into the 50a outlet on the adaptor you made, and you will have a normal 30a/120v connection to your RV.
This is mostly correct but you are wrong about current. You can not get 120V/60 A out of a 30 A dryer circuit.
A four wire dryer circuit has two hots, a neutral and a ground. There is 240 V between the two hots and 120V between each hot and the neutral (and ground).
You could use a 240V/30A circuit to power two different 120V/30A RV outlets but you can not get 120V/60A on a single outlet from that circuit.
If you plug in 120V/50A in to a dryer circuit, it will trip at 30 A. If you really want a 50 A circuit you need a new 50 A circuit breaker. You could wire to a 40 A/240V four wire stove circuit, but the breaker would still trip at 40 A.
The fire hazard and danger is if you have a 240V / 30 A three wire circuit. This has two hots with 240 V between them. There will be 120 V between those hot wires and ground, but the ground wire is not meant to carry the 30 A current. When you plug in a 120 V / 30 A load and try to use the ground wire as a neutral, it will overheat without tripping the breaker, causing the hazard.
The only safe way to do this is have a four wire 240V/30A branch circuit with properly sized hot, neutral and ground wires all the way back to the 30 A circuit breaker. For example if you see two #8 hot wires (red and black) out of your 30 A breaker and a smaller #10 (white) then it's not safe. I don't think romex/loomex ever sold 8/3 wire like this but if your house has conduit there's a chance they cheaped out on the neutral wire.
If you have a properly wired dryer circuit, all you need is a dryer cord, an RV outlet and a properly sized electrical box and some bare copper ground wire for the box. Connect the green ground wire and white neutral wire according to the outlet markings and then connect *either* the red or black hot wire but not both to the RV outlet. Take the remaining hot wire and put a wire nut or tape it with electrical tape and tuck it out of the way so it doesn't short. Put it all together and check that you have 120 V and that the outlet polarity is correct (hot and neutral aren't swapped). Inside the RV, one of those plug in testers will tell you if it's properly wired too.
If you wire this up correctly you probably have less risk to your equipment and safety than by using a 120 V 15 A circuit, RV adapter plug and undersized extension cords. Low voltages can damage equipment and overheated extension cords and 15 A branch circuits are a fire hazard too.
