What area of ND are you in? im guessing central but not sure.
if your parents are able to pour 40-80 pounds (i dont know how hard the house is to heat) of anthracite into a hopper or stove door each day, and remove a pan of ash each day, maybe either a stoker or hand fed with/without hopper anthracite coal stove might work. Even way up there, anthracite would likely be a little cheaper than propane. If you bought a semi-load it would probably still be less than 350/ton. No shoveling as it is in 40-50 pound bags on pallets. Its a whole new ballgame than lignite. 40 pounds a day average will heat an average sized/insulated house in our climate with a decent stove.