A 30 amp service can only supply 30 amps. If the current rises much above 30 amps the breaker will trip. If the equipment is properly constructed and in good condition it will not over heat at 30 amps or some design amount above that. Low voltage cannot make the current increase more than that 30 amps without causing the breaker to trip. And the RV cannot overload the circuit without also tripping the breaker. Again, if properly constructed and in good condition.
Heat is caused by too much current flowing thru a resistance. A bad, worn, or loose receptacle will cause the plug to overheat and possibly melt the plug. Likewise a bad, worn, or loose plug will cause the plug to overheat. A loose wire in either will do the same. All of those add resistance to the circuit.