Good questions and a great reason to start with 2 or 3 coats of epoxy primer...
For one, the reflectix isn't water permeable so no absorbing and holding moisture against the meta/paint like Bill was pointing out and your 71 demonstrated.
Second the temp difference between the cab interior and the metal of the cab is less (by being insulated)and the metal doesn't act to condense the moisture in the air that the jute absorbs that the dirt sticks to where the rust grows and gets a new home E-I-E-I-O.
