We may have an option for you, though the part you asked about is unable to be used in the rise position, it can only be used in the drop position. For flat towing, you can use a high-low adapter in the rise position, and can do so without being perfectly level; what you need is to be within 3 inches of level, as you can see by the diagram I added.
So with this in mind, the adapter with the most possible rise is 10-1/4 inches, and that will work if you are within 3 inches of being level:
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The MaxxFan Deluxe Roof Vent w/ 12V Fan # MA00-07500K referenced in your question fits a 14 inch x 14 inch RV roof opening, but the mounting flange itself measures 16-1/2 inches x 16-1/2 inches. This will refer to the footprint of the unit from inside your RV/trailer.
The actual vent that will mount above the roof of the trailer measures 22-1/2 inches long and 16-1/2 inches wide. When closed it will extend 5 inches above the roof and when open it will extend 9-5/16 inches above the roof.... view full answer...