Getting ready for another road trip (yes, in December)... I think I may have the gasket sealing properly around the AC now, used a jigsaw to get a slightly larger opening in the roof, which should allow more of the gasket to seat into the opening, trimming the sides with the widest margins.
Still have one corner that isn't compressing quite as much as I'd like, but we'll give it a go and see what happens. Not expecting any rain this trip, more likely to be snow... May not know till spring if this has done the trick or not.