Well it depends on how you define caretaker. As the decision maker responsible for Spence's care, then yes, she was primary. But Pru had a lot of help and despite challenging financial circumstances, she had much more help that the vast majority of people trying to care for a loved one in similar circumstances - a day nurse and a night nurse - and still we see how the wear and stress take their toll on Pru.
It's interesting that the apartment they bought, at Pru's urging, with Spence's book advance windfall from so many years earlier, helped to finance his care. Even more interesting was Arlo's action in this regard - don't want to be explicit for fear of spoiling.