Hi and welcome to the forum
According to Dr Googley, the temperature in winter can drop below freezing in Oklahoma. Under these circumstances it is probably a bad idea to leave the aquarium outdoors unless you can greenhouse the area that the tank is in. Even then you will need to put a lot of insulation underneath the aquarium, as well as on the sides and top, and have heaters running in the tank.
The easiest way to do this might be simply building a frame around the tank and stand and insulating that with 4 inch thick polystyrene or some other type of insulation. You would need insulation on the floor too and seal up all the corners and gaps to stop cold air getting in.
Basically you would be building a small enclosure/ fish room for the aquarium and stand, and insulating the enclosure. The aquarium and stand would be inside the enclosure and you would have a couple of aquarium heaters running to keep the water warm. I would look at using two x 300 watt heaters. Two x 200 watt heaters would probably work as well.
You would have to have good coverglass on the tank to minimise evaporation so the humidity doesn't build up in the enclosure and short out the light unit.
Each day before the lights come on above the aquarium, you would have to have a small low wattage light come on or remove a section of panelling so light could get into the tank. Alternatively open the door of the enclosure and that would allow some light in. Then wait 30 minutes or so before turning the tank light on and closing the door.
In the evening you would need to have a low wattage LED light on the tank and then turn the tank light off. Wait 30 minutes or more before turning the low wattage light off. You could probably even leave the low wattage light on all the time.Doing this will reduce the stress on the fish and they won't be going from a brightly lit tank to complete darkness and vice versa.
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Another option would be large plastic storage containers kept inside. You could ask your landlord (I assume you're renting) if you could keep the fish in a large plastic storage container indoors. Then put the fish in a large plastic storage container and keep them inside over winter. If they have concerns over water leaking, put one container inside another container. Keep the fish in the smaller container and if that breaks for some reason (it shouldn't but might), any water leaking out would be confined to the larger container that the small container is sitting in.
You could also use a large plastic storage container and keep the fish in that and when the rent inspection is due, remove filters and airlines, etc, have the lid on the container and put some stuffed toys on it to make it look like a toy box.
I knew a guy that built a wooden frame and put a plastic storage container in that for his fish over winter. During rent inspections he removed all fish related equipment and put a wooden lid with a vinyl covered top on the box and it looked like a seat. He built it sturdy enough so you could sit on it. It was insulated and just looked like a piece of furniture.
If worse comes to worst, wash the bathtub out really well with soapy water (use a perfume free soap) and rinse well. Get a new plug so it doesn't leak and put the fish in that. But make sure it is washed out well before adding fish and don't let chemicals or sprays get into the water. It's risky to the fish but might satisfy the landlord. When winter has finished you can move the fish back outside and bleach the bathtub.
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On a side note, if you do move the fish indoors over winter, empty the water out of the aquarium so it doesn't freeze, and give the tank a good clean. It's probably a good idea to put the empty aquarium somewhere safe so it doesn't freeze as well. I am unsure how well silicon lasts when frozen. You could contact a company that manufactures aquarium silicon and see if the silicon is going to break down or be damaged when frozen.
