Dave, I don't think you want that first part of your loop because if you will tend to condense vapors over time and they will settle in that low spot. I believe your hose should slope up from the vent connection to your high point, then perhaps turn down. I still don't see any value in having it drop down to tank level, but I don't think you want any low points in that line.

I think I will add a riser like yours to my trunk mounted tank, just for grins. My vent line will run up to the top of the trunk, only about 6", then drop down to a drain fitting through the floor.