Electric heaters (try it out in the store to make sure the noise doesn't bother you) and if it's predicted to be REALLY cold, get 2. One will need to be plugged in directly to your power pole at Ft. Wilderness via a heavy duty (not household) extension cord (you can run it up through the canvas/vinyl where it's velcroed or bungeed into the inside of the camper. The reason for 2 separate lines if 2 heaters is it will trip your breaker otherwise.
You don't say what year etc your camper is, but most now are 30A and the older ones were only 15A. The power poles at Ft. Wilderness have 50A,30A and 15A on some not all . (never a problem no matter as hockey puck adapters (nicknamed for their shape versus the dogbone adapters (so called for their shape also and considered safer and better ) are okay for a few days use and cheap, available lots of places including
Walmart. Just do not use a 14 gauge regular extension for the second heater.
If you want to really keep warm, you can't beat also adding Reflectix material inserts to your windows and (some say yes some say no on this next item) reflective covers on the bunk end tops reversed with the shiny side down.
As always, you will still need a small opening (couple of inches ) on each bunkend end window to avoid condensation if it's cold (the air you breathe out is warmer ergo condensation) or small fan circulating does the same.
There are also very effective heaters called Mr Buddy . One size takes one of the small propane cylinders. Another size they offer takes two. You are safe in the popup camper to run these because no popup is totally airtight of course and the new models have safety tipovers and oxygen sensor warnings.
Do not put any heater of any kind on a soft surface such as a cushioned seat or carpeting.
There are several websites dedicated to camping with specific areas for popup campers and complete websites dedicated to popups as well. Since you have one, you will find them of interest most likely not only for this situation.
Check out popupexplorer [dot com], popuptimes [dot com] and others; just do a 'search' the first one listed is considered to be the most helpful by some (you do not have to register to read but do have to register to post)
Do not use an electric blanket if you have a pet.
There are many choices of sleeping bags rated to -40 degrees effective and many more options I would suggest, but these so far should give you the answers you need I believe.
We spend considerable time several trips each year at Ft.Wilderness and 5 years ago in January, a young couple camped in an old popup next to us knocked on the door of our RV around 8pm and said they were just wondering if we had an extra blanket or two they could borrow as the weather had unexpectedly gotten much colder, indeed it was downright very cold by FL standards for a few days then. Even though our bedding and furnace were the most we had ever needed for any time we needed heat, we did keep 2 sleeping bags in storage in our RV for grandkids visiting and the like.. We gave those to the young couple and one of the two 1500watt electric ceramic heaters we kept stored as well for extra use if needed for emergency(electric out/propane out emergencies) The next morning early around 7am we took our dinghy and left for the day to do some things. The couple next to us appeared to be still in bed since their popup and tow vehicle, a van, were still there looking the same , and we had assumed they were warm with what we gave them to use for the night or nights that they needed since they didn't knock on the door again. We returned to our site around 6pm that night. The young couple and their popup were gone. And so were our sleeping bags and heater. My wife and I still laugh about it now years later, as all they had to have done was come to us and say thanks, that really helped us , we'll have to get a heater like that and maybe warmer sleeping bags -and we would most likely have said Keep them, we'd like to help you and it's not a big deal. But stealing is a different issue but what goes around usually comes around somehow.