a dip in the line is always going to have fluid sitting there partially contaminated.
The weight of the fluid must cause it to dip and over 8 years, its stays that way. I may consider coupling in an ABS pipe for just this part of the run, or come up with a bracket of some kind.
It may be a good time to ask the question, while we are thinking about this issue.
Is it always best to ensure there is no dip?
I understand the matter of hose permeation, but the dip, if deep enough to completely fill the hose, will function like a sink trap to block sewer gases. This can be needed more on a boat than in a house because the vent on a boat is at times aimed right into the wind, and the resulting pressure can force gases out the head.
One winter, I diligently shortened and straightened the head hoses so there would be no dip at all. Next season, on occasion my wife would comment on the head smelling worse than usual. I soon figured it out from the wind direction, but was reluctant to admit to creating the problem, and also wanted to remain "dip-free". Next haulout though, I installed a second vent on the opposite side of the hull, so I no longer have the problem, even with no dips. Plus, the tank gets a good flow of air.
So beware if you try to remove the dip!