I presume the plumbing for your '88 Maramu is the same as my '82. It will be easy to determine... Pull up the floor boards on the port side of the saloon table. If you find a fiberglass bulge running along the side of the encasement of the water tank, then we have the same plumbing. The copper lines for the forward head run inside this bulge/channel, through the bulkhead, under the shower pan and terminate at the sink. The lines feed into this channel at a junction under the refrigerator.
Amel realized that this plumbing would be virtually inaccessible for maintenance which is probably why they made the effort to encase it in fibreglass. Unfortunately for you and for me, the copper has finally corroded and is leaking. In your case, the solution may only be to remove the shower pan to access the leak. My problem is at the other end where the copper enters the fiberglass channel.
I'm not familiar with the process of removing the shower pan but I'm pretty sure that while it's possible, it's a major project requiring a lot of head parts removal and maybe even some demolition. Perhaps some other Maramu owner can help with guidance for that.