I think your only viable solution long term is to bring them inside as in a garage or basement, and yes, insulate the tank and use a heater. I use one or two aquarium heaters in my 300 gallon recirculating systems and they do the job keeping temps up in the 70's even without insulation -- even though the air temp in the basement is 64 in the dead of winter -- (water temp will drop two degrees below ambient temp.)

Considering how cold it got in Oklahoma last winter your outside option is finite.

If you use a heater I'm not sure I'd trust a Chinese model inside that is probably not UL listed. Or at least use a name brand heater that is UL listed even if it's made in China.

BTW if you want to consider a recirculating system you could look into an upflow sand filter used for large aquariums. They are not too high in price and have a small footprint. All you need then is a clarifer tank which you can make out of a fifty five gallon drum and some deer fence netting and some PVC and you're good to go on the inside. Of course you will need to cycle your tank first.

If pigs could fly bacon would be harder to come by and there would be a lot of damaged trees.