There's not much in the way of options for it between ducking below the frame rail and having slope for positive drainage on the long length needed from the grey tank.
Members in our camping club have one, and never had any issue with obstructions. However, a blowout of the left driver's side rear tire (another story altogether but it's having axles replaced) took out all of the piping and the valve. Luckily, they were on their way home from camping! They had, and I think wisely, re-plumbed using separate valves for the black and grey tanks and now they are tucked more tightly to the frame rail.
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