Compare the electrical to your water hose. Amps is quantity (in this case available), Voltage is pressure. The 50 amp rating is overcurrent protection, your circuit breaker. The circuit breaker is the gate keeper. It is best to match your load to the rating of the wiring, like 30 amp or 50 amp. It is okay if you use a 30 amp cable on the 50 amp service because at your RV the circuits have breakers. If you try to draw more than the 50 amp quantity all at once the 50 amp rated circuit breaker will trip. If you connect to high voltage there is too much pressure and it bursts your electrical stuff (lets the *smoke out). Watts is the way your consumption is measured so cost is based on what you consumed (electrical energy converted to work).
(*smoke is what makes all of your electrical stuff work. If you let the smoke out your stuff will stop working)