regardless if your RV has 50a service or 30a service, the common 50ft 16ga extension cord that people typically buy to plug their RV into a 15-20 amp receptical, can barely deliver enough power thru it to run a space heater. its running at the very upper limit of the safety margin..
a 16ga ext cord does a great job for supplying power to a charger only, for the 12v circuits.
but if you are going to run a space heater, and want enough power left to run the fridge, water heater or charger, you should at least use a 14 or 12ga extension cord, and its preferred to have it plugged into a "heavy duty" 15a outlet, which is properly called a 20amp outlet...
30a power is much more preferable for every reason, because a 30amp cord has 10ga wires and can carry the voltage easily and safely.