My auto water change DOS is set up in about the same way. Probably 12 feet up my wall, 30 feet to my garage and then down 6 or 7 feet.
If the Mur-Lok connection is leaking it’s possible that it’s a bad fitting, but I’ve definitely seen cases where I didn’t get the tube pushed in all the way and that causes leaking. If you haven’t yet I would try to disconnect and reconnect the tube. Make sure the end of the tube is not damaged and it has a clean straight cut as well. If there is any damage or if the tube is cut at too much of an angle that can cause leaking also.
My auto water change has been in operation for a few years now, but I do remember that priming took a long time. You can push the button on the unit to prime it or just set the outlet manually to “ON” in fusion. Note that when the outlet is turned ON manually the DOS will run for a bit, then shut off for a bit, then run for a bit. This is normal and is done to make sure the pumps don’t overheat.
Unless something has changed recently, if you set it up the standard way for the OSW pump it is configured to run the pump backwards during the water change operation, but the prime button will run the pump forwards. As I recall what I did to prime the OSW line was to put the exit of the tube into a container of clean water so that when I did the priming it would be able to suck water into the tube. Obviously I didn’t have the end of my tube in my tank during this operation. Then once water finally started coming out the other end I knew the line was full of water and I put the correct end in my tank and the other end in my drain.
Finally, if you have any additional leaking Mur-Lok fittings, especially on the suction side, this could prevent the line from priming as it may be sucking air in through the leaking fitting.
Good luck