Use the biggest gauge cord you've got. The skinnier they are, the more the risk of overheating. Also. it's the length of the cord that's important, not how far you extend it. A 200' cord has the same characteristics even if it's coiled up and only runs 30'.
I have that situation. If the RV is out in the street, I need most of the 150' cord I have. If it's in its parking slot alongside the garage, the run is only about 15'. However, it's the only 8-gauge cord I have - the short ones are skinny little things, maybe 18 gauge. I normally only run lights occasionally and keep the batteries charged, but when winterised, I put a 1500 watt oil-filled radiator with a plug-in digital thermostat which is set up to keep the interior at about 38F. We don't get really cold here, less than 25F for a winter low is quite rare and daytime highs are almost always above 32F.
Of course, summer highs are rarely above 75F, but on the whole it's a pleasant climate, except for the "Indefinite ceiling, sky obscured, 39 degrees, rain and fog" that our local airport ATIS often gives out in winter. A couple of weeks with every day like that can get up your nose a little!
Frank and Eileen
No longer RVers or FR owners