I travel a lot for work. I'd really enjoy another freshwater aquarium but I don't want to heat my home when I'm gone for a week at a time. Can those tank heaters do the trick when the air temp is 55 degrees or so?

i believe so yes
But are they meant to be kept on all the time?
I'd worry about keeping them on for weeks and weeks at a time

buy a timer and run it at various times---and buy a GOOD heater,dont skimp