I once oversaw some "heat aging" tests on Portra's immediate predecessor, VPSIII. Basically, the film spent time in a "hot box" at 140 deg F, then was tested at various intervals by exposing a sensi wedge, which was processed and evaluated.
As I recall, the VPSIII showed no changes at all, until somewhere around 300-400 hours, at which time it started going downhill at a steady rate. This behaviour was nothing like you would expect based on Internet lore.
My guess is that Portra film would have similar (or better) characteristics, such that 10 hours or so, possibly reaching 100 or 110 deg F in your trunk would have a negligible effect on the film.