My main thoughts are, energy is finite. If it's storing it in potatoes, it's not giving it to the fruit.
I can think of a graft that wouldn't have this problem, though - "tomacco", tomato/tobacco, or "potacco", potato/tobacco
If you grafted tomato vines onto a tobacco stock or tobacco onto a potato stock, the tobacco leaves at the base would continue to be sugar producers, not consumers. You get you tobacco leaves and you also get your veggies. Though I have to wonder whether the nicotine would be sap-mobile...