mystery leak

On our last trip I noticed that our 2011 3124KB had an occasional, slow leak. It was leaking from the bottom trim piece around the external shower (see photo 1), but the ext shower seemed just fine. The leak was slow and I didn’t notice it every day. I had previously noticed that items in our external rear storage were damp.

After landing back at home I tried to reproduce the leak. No leaks when using the fresh water tank and the pump. Hooked it up to city water for days - no leaks. Ran all the faucets, cold and hot taps - no leaks. Thought maybe it was the shower given the proximity, but no leaks in the fresh water nor from the drain.

Figuring it had to have been water from the shower that somehow ran behind the splash board (which I did have to re-calk), or the leak self healed for a while (wishful thinking), I went on to the next task which was replacing the anode rod and flushing the water heater. Finished that and proceeded to test the hot water heater. The hot water heater works just fine, but the leak is back. Now the water is coming from right where the bottom trim piece meets the wheel well.

Now I’m suspecting the hot water heater, though it would need to have a slightly bizarre path to leak at the rear portion of the wheel well. Crawl under the kitchen sink – nothing. Under the bathroom sink – nothing. Confirmed again that the leak only happens when the hot water heater is running – electric or gas. Once the water is hot and the heater is just sitting idle, the leak eventually stops. ..and when it’s leaking, the water is not hot.

After several more trials and contorting my body to get a view of the plumbing behind the exterior and inside showers via the “service” hole in the rear cargo compartment (photo 2), I spotted a the drip. It looks like it’s coming from exterior shower connections (90% sure). I’ve noticed other posts on here that indicates the ext shower as a point of failure.

So…..
Any theories or similar experiences with why a leak might occur there only when the hot water heater is running? My thermodynamics knowledge is not so good! I would have thought being hooked to city water (I do use a pressure regulator) regardless of the heater being on would make a leak deterministic.

Second question: Any experience on repairing a leak behind the external shower? There’s no way to get at it via the rear cargo area. Looks like I’d have to remove the entire shower to get to at it from the inside. It almost looks like a hole was cut and later patched in the TM underside to get at this area (~ 1 sq ft area encompassing the 2 drain valves.) Hopefully there’s enough maneuvering room if I take out the ext shower itself.

Appreciate any guidance.
-mike