I don't believe Amel put holding tanks in SN's so, like ours, yours was likely done by a prior owner. (Just the forward was done when we got our boat - I later did a 2nd tank in the aft head.) From your question it sounds like your tank is behind the outboard panel with the openings for the storage shelf. If so, removal of the wall would depend on how whoever installed the tank did it in the first place.
On mine, it requires removal of the sink wood crossmembers for the towel rack, the medicine cabinet, plus some perimeter trim, then reaching inside the cubby openingings to remove a few screws. Not too difficult. The tank simply sits on one of the original shelves and the plumbing is readily accessible once the panel is off. I redid mine with PVC which I'd highly recommend for a multi-decade no-odor fix.
That being said, I recall several posts of other SN owners installing tanks in various other positions, like under the towel bars. What's the configuration on your SN?
PS I forgot to mention the original panel was fiberglassed to the shelves, so that has to be cut open the first time the panel is removed. Upon replacing, use wood blocks and screws to fasten it. A FINE Multi-tool or equivalent will cut the fiberglass tabbing nicely.