I live in MN as well and has never had to use heat lamps. I have used heatmats on the side of the tank before but now that I have all my frogs and fishes in their own room I just run a room heater.

BTW these frogs are not from the swamps of Africa so IMO you don't have to worry about humidity, at least I don't. If you look at the videos and pictures of where these frogs are from, these places are usually dry places with just a small puddle here and there and they all dry up very quick.