Agree with the above posts. Educate yourself on your battery and how to not "waste" power and you'll be fine. A couple days on one battery should be ok if it's in good shape and not abused. If your furnace is fan forced, then don't let it run overnight as the fan will deplete your batteries.
Two batteries is, of course, better. However, if your current battery is old and tired, don't go buy a second to run in parallel as pairing an old tired battery with a new battery will decrease the new battery's life severely.
A generator is a nice thing to have, but unless your onboard charger is good, then plan on running it for a few hours to replenish your batteries. We stayed in the park for a few nights and really enjoyed it.
Best of luck. Enjoy your trip.