Hi,
The ceramic heater will not care if it is running at 120 volts or 95 volts. It will just draw less amperage at 100 volts than 120 volts (20% less amperage) and also will put out less heat. So if rated at 1200 watts at 120 volts, it would be drawing 10 amps. At 100 volts it would draw only about 8 amps, and make 800 watts of heat.
If you have a modern electronic battery charger, it should not care if it has anything from 95 to 135 volts. They are very tolerant of input voltage.
I would suggest that #12 wire is minimum if you will run a 12 amp heater. Otherwise a smaller cord can be used if just running the battery charger or TV and computers/ cell chargers. A coffee maker can use 12 amps, and thus requires the #12 or larger #10 power cord.
Likewise the coffee maker will take more time at 110 volts to make coffee than at 120 volts. You can help this out by warming the water on the stove to about 95F before putting it into the coffee maker.
Your furnace should be used to warm the RV to 65F or so in the mornings. The ceramic heater can not be run while using high amperage items such as the coffee maker or running the water heater on electric.
You will need to either keep the RV winterized and not use the plumbing system, or keep the water heater warm by running it enough to keep it above 40F at night. The easy way is to just leave it on and at 140F, but that will waste gas, as the cold air will go in the bottom and fairly quickly cool the water with 10F air going in and being warmed to 130F and flowing out the top of the flue. What I do is run the heater for a 1/2 hour or so, then shut it off, and hope it will not freeze overnight. Then run it for 10 minutes or so to keep it warm all the time.
Have fun camping!
Fred.
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