Thanks for all the responses.
Further info: we're in the Fairbanks, AK area. They're in a low spot, so lots of -40 weather this year.
The water tank is for household water use, delivered by tankers, and buried. Agreed, Dix, that in-the-basement tankage has a lot of advantages (except when they age out and have to be replaced--been there, done that!). However, they don't have a basement, so like many houses, this one has a buried tank. I've never heard of a buried tank freezing, either, and it makes me kind of nervous to hear that this is possible, as mine is also buried. I don't know how deep their tank is buried, or whether it was insulated. This should not have happened.
The access to the tank is through a fill tube, probably about 2-1/2" in diameter, so those other heaters won't work. They used to make a copper tube heater that could be poked down into tanks, but alas, someone sued them for something, and they are no longer made. And I've lost mine in a move.
They heat their house with a boiler and a Toyo oil heater, and probably have an electric HW heater. Pretty standard fare, at least around here.
I called them up last night to offer the use of a handpump I have that fits in a fill tube (I bought it in the event that I lose power and still need to access water from the tank) and I was told that they rented a 7-hp pump to empty out the tank. Haven't spoken to them yet this a.m. to see how that went.
Dune, that block heater idea is a good one. That might be small enough to fit in the tank hole.
Dang, the adventures never cease. Thanks again, all, for your ideas.