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Is your e-brake adjusted properly? I have an E4OD, but the e-brake should hold, if properly adjusted. Of course, a significant grade is an entirely different critter, but for most parking situations, the e-brake should be fine, even with the trailer connected.
You might want to verify if the trailer brakes applied when the power cable is disconnected. If they are, unplug the trailer from the truck when parking. Extra insurance and peace-of-mind.
I have a auto tranny, but I agree the e brake does nothing. If you put the truck in neutral and put the e brake on it goes right through like nothing. The Excursion does the same thing. If you were parking it for a while, i would use wheel chocks, no such thing as being too safe.