Posting this for posterity in case someone else has a similar problem.
I had a leak from the shower of my 27FB. Checked the pressure side from the wardrobe access panel and all is dry. Water would drip from under the access panels. Tested drain by bringing in a bucket of water to pour into the shower which then would leak. Fortunately, the water would leak only into the bellypan and not the interior floor.
Problem was the p-trap. Collar attaching the p-trap to the shower drain became brittle over the years and failed.
Access for a 27FB cannot be done under the belly as I found out. The plumbing is behind the banana-wrap outside of the frame rails. Fortunately, the 27FB interior has a furnace hose conduit between the end of the bed and shower closet. Take cover off, and pull furnace hose back under the shower. Reaching blindly into the little 5" hole there, I was able to remove the P-trap by feel and unscrew both collars.
Here's the pieces necessary to rebuild the p-trap:
I used this kit - https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1
Also needed a short bit of 1.5" pipe, and 1.5" pipe coupler