Depending upon the model you have, you may be able to remove a bulkhead in the storage compartment. From there you may be able to see plumbing from under the shower and/or plumbing leading into the holding tanks. The shower drain probably has a component incorporated in it that serves as a water trap. Make sure it is tight. Run the shower while looking around in there. This is way easier than dropping coroplast for diagnosing a shower leak. Good luck.
2016 3160, Legacy, Sailuns, Splendide 2100 xc vented, 1 1/2" axle lift blocks, 2014 Ram 3500 SRW SWB 4X4 6.7 Aisin Mega Cab, EBC slotted disks and brakes, Titan fuel tank.