Dicor and 303

I would not try this for a couple of reasons. First the 303 might damage instead of help extend the life of the dicor. Dicor lasts for a long, long time. If the Dicor fails the reason is most likely due to flexing and cracks opening in the RV itself. Finally, 303 has a very short life outdoors. Any positive benefit vanishes within a few weeks as the 303 washes and weathers away.
Of course, since you live in a dry climate, your experience might be different. My experience in a wetter climate means I just don't bother to use it on any external surfaces.