I'll be spending 2 nights at the onion valley campground. Hopefully this will help.
That is PERFECT! And high enough that your two nights at OV "count" toward your acclimatization.
That is exactly what I did before my first Whitney summit, in 2010 (two nights at OV, plus a day hike up to Kearsarge Pass). Trip report
here. You might consider, on your "off" day, a nice, slow, acclimatizing hike up to Gilbert Lake, maybe a side trip to Flower Lake. Gilbert Lake is big, and there be trout if you fish. Kearsarge Pass is great, though you risk tiring yourself out if you climb that far, particularly with your prior trip to White.
