You gotta tell us how much the voltage drops to before we can say if it's harmful or not. Dropping from 120 to 110 is no problem for example. Dropping to 90 is a problem. Also, measure at the source; where the RV plugs into. Is the voltage drop at the source so all electrical in the RV is dropped, or is it on the circuit the heater is on only. If the latter, then it's just undersized wiring in the RV to run that heater. Find an outlet that has a higher rated breaker as that will have heavier gauge wire and less voltage drop. If it's at the source then you may be plugged into 50amp circuit but you sure aren't getting it. An electric space heater has a typical amp draw of 12 to 15 amps, leaving you a reserve of at least 35amps for everything else.
