In general, a modest voltage drop in response to a significant load is nothing more than evidence of Ohm's law in action. If it's a volt or two, it's nothing at all to worry about.
If it's several volts, the lower voltage is not in and of itself a problem in most cases (provided it's still in the right nominal range, not much below 110V), but the amount of change suggests a relatively high resistance between the heater and the power source. That could simply be long wires in the campground and not a real problem. It could also be a dirty plug or loose connection somewhere that's getting quite hot due to the power its dissipating, and that is very much a problem.
One good thing to check is to see if the voltage at the other outlets in the campground electrical box is drooping and how much. If you see an equal drop there, then the cause is in the campground wiring and there's not much you can do about it. If the voltage remains pretty much steady, then check your connections carefully: see if the plug is warm, check (with power off and disconnected!) that the wires are tightly connected in the breaker box for the RV and none of them look overheated, etc.
Of course, the 30A and 20A receptacles in the campground power box are only single phase, so measuring the voltage at them only checks one of the two legs. Often they are wired to opposite legs so checking both can verify both legs, but I'm sure that's not universally the case.
