You can get by with a standard 16 or 14 ga. cord if you're only running converter to keep battery charged, and to run low draw items such as lights, water pump, furnace, and fridge (on propane). Do not use high draw items such as air conditioner, microwave, electric hot water heater, or any portable electric heaters. Best bet is a heavy duty extension cord, which is 12 or 10 ga. I still would not attempt to use air conditioner on any extension cord. A very real fire hazard! I just keep a battery tender hooked to the battery all winter. This way all the high draw 120v stuff is offline anyway. Only the 12v stuff is available. Length of cord is also an issue. The longer the cord, the heavier it must be. Try to keep it under 50'. There are some you tube videos on extension cord safety with RVs.
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