My previous B+, a Starflyte, did not have waste tank heaters. I used a half a gallon of antifreeze in each waste tank (and poured a little more in the traps before bed each night. I also added a gallon of antifreeze to the fresh tank (we DON'T drink it, use bottled water exclusively). We were out in one snow storn in Massachusetts with it and survived okay. None of our experience was with sub 20 degree weather however.
My new B+, the Trail-Lite, we got tank heaters. Had one night in the upper teens this November and didn't worry once.