Sounds like your house might have a Michigan basement and the rubblestones have moved a bit to let one wall drop or the center beam is sagging to let the floor move. The "cure" for your floor slanting is under the floor.
Those gaps are part of wood floors in the winter. The houses heat causes dry conditions for the wood. Wood moves with changes in humidity actoss it's width. It moves among it's length but a lot less. In any case, it you have anything in the gaps, and this includes a filler or dirt, when the sood expands in the summer, some of the wood cells get permanently crushed. When next winter comes and brings dryer conditions again, the gaps are bigger. Obviously this is not good, so don't apply a filler.
In the summer, as long as there are no visible gaps that let you see into the basement, or from the basement into that room, and if there is enough wood thickness to sand, you can refinish the floor.