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.