Let's dig into this a little deeper for future reference. Anyone feel free to correct me.
I recommend a 50ft 12AWG extension. This will ideally produce a 3V drop at 15A. The adapter will produce about a 1V drop. Considering 120V at the outlet, the camper should have 115V while drawing 15A. That's 1725W. I like to try to not exceed 1700W while on this extension chord.
Using conservative numbers to account for the converter:
Furnace = 100W
Fridge on AC = 350W
Water heater on AC = 1500W
Microwave = 1300W
Standard light bulb = 20W each
Using these numbers, you can operate your furnace, some lights and the fridge on AC without coming close to the 1700W allotment. You can operate the furnace, fridge and microwave for a short period of time. You can operate the water heater or the microwave, but not at the same time. It's called power management.
Forget about the air conditioner. I do not recommend operating it on an extension chord.