Ian,
I will be checking BOTH!
My pump comes on periodically (sometimes after 4-5 hours) for a fraction of a second. Because the leak would would have to be so slow to cause this, and since I have no water anywhere, I didn't even know where to start without some crazy isolation process.
But, the aft shower is a perfect place to start. I may add some shutoff valves to confirm this is the issue and then leave them in place since we don't use the shower that often. I have had concerns since if there was a leak we won't know it until all water was just about emptied from the tanks due to the location.
Also, do you know where to source these hot/cold lines? West Marine has soft lines with reinforcement which make the outer surface not flat possibly causing leaks with the fittings.
Thanks again Ian!
Stuart