The Thing-in-itself is perhaps not straightforwardly, wholly, and unconditionally will.
The Thing-in-itself is the enormous error that there is a true reality behind appearances.
The Thing-in-itself is will, but, it must be qualified that it is only 'to us' that it appears as will, and it could have other modes of being that are incomprehensible in ordinary terms, but which might be accessible to mystical consciousness.
The Thing-in-itself is perhaps not wholly identical to the world as will, but it nonetheless includes as its manifestations, the world as will and the world as representation.
The Thing-in-itself is pace kant, something we can in fact gain knowledge of. The relationship between representation and thing-in-itself can be understood in analogy to the relationship between human will and the human body.
The Thing-in-itself is what we wrongly think we are referencing when we speak of trees, colors, snow, and flowers. But these are mere metaphors which correspond in no way to the original entities.
The Thing-in-itself is, despite what Kant says, that about which knowledge is not wholly impossible. It may be available to mystical consciousness.