I'll take a stab at some things that normally I would check,
like the hot and cold water lines under the sink, and behind the shower wall, especially plastic pex connectors where or if you have flex hose connected joining to hard pex lines for the sink and shower. Then I would inspect the drain plumbing of the shower for loose connections. Or a cracked shower pan floor especially around the drain. The shower pan could be cracked in a certain place and when you step on that spot the pan leaks.
Also most of the time in these RVs, the black tank rinse lines run up behind the shower wall to a check valve, then back down into the blank tank. The check valve itself or the lines running to it might be leaking.
Even the hot water heater lines in and out and especially the plastic check valve on the back side of the water heater, those cause a lot of owners troubles.
But then you said it doesn't happen when taking a shower. Only when it rains?
Well that kind of eliminates those trouble spots I mentioned.
Happens when raining, well yes I guess the water heater exterior side wall seal and panel could be leaking when it rains. I think I would also get up on the roof and look at the vent line caps and seals. This is where the drain plumbing vent lines for the holding tanks go through the roof. I think I would also check the skylight for cracks and seals,
Of course water can leak from somewhere, then run over to another place where then you can see it. That makes it hard sometimes to find and back track to the source. Is there any kind of inspection panels you can remove to get inside of the walls or under the shower pan? In our old Imagine 2950RL there was an inspection panel I had to remove to gain access to the back side of the water heater to turn the water line valves in bypass mode for winter storage.
Hope I gave you some ideas and things to think about and check. Sorry I couldn't be of more help, I'm not familiar with the current 2970 layout.