I have run two space heaters regularly in my 30 Amp Adventurer 35U for years and have no issues along with no second extension cord. The worst heaters were ceramic since they ran almost constantly with few having a way to limit how many watts they consumed while the cheapest heater fans or wall hangable portable space heaters that had nickle wire heating elements in them work the best since most will allow you to run them at a 400 or 600 watt setting. The most dangerous are the parabolic heaters with their mirrored dish and bright cherry pepper red hot elements.
I would try an inexpensive heater fan running at its lowest wattage first and then decide if you really need to run a second feed to use the sometimes available 20 amp convenience outlet that some campgrounds have on their power poles. Note that at too many campgrounds this 20 amp breaker is not on a separate feed and draws off the 30 amp line so you can still get power drops and breakers tripping outside instead of inside your trailer.
As for running a second wire then follow the power into your trailer and use that same route if possible.
Hopefully this is not your situation
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