Thank you kindly for the good responses. I need help in this one and I appreciate what you folks have suggested.
I had an idea today. I decided to drain my system (think winterizing) and then pressurize it with 35 psi air pressure. Then I got my soapy solution out and I found the exact spot of the leak. It was the upper tee after all ! And it leaks badly. Blows big bubbles. The leak radiates from the frame rail side, right at the joint between the poly tube and fitting, just ahead of the clamp.
I was surprised that city water pressurizes the output pressure line from the 12v
pump. The pump is the "check valve" preventing water pressure from entering the fresh water tank. My Trade Wind has a check valve downstream of the pump preventing city water pressure from entering the pressure side of the pump.
So I cleaned thoroughly with alcohol, towel and a stick and applied a 2 part epoxy designed for plastics. I made a little "hook spatula" to work the epoxy to the side and bottom of the fitting. I cannot reach this fitting with my fingers. This is a non sagging epoxy, but probably doesn't wick very good. I'll let it cure overnight, and retest it with air pressure tomorrow. I'll apply again if it is still leaking. If I can slow it down some, I'll take my planned trip with a leaky pipe. The water is leaking down the inside surface of the frame rail into the fresh water pan, and out the access port. Nothing is at risk of water damage. I'll fix it this summer when I can dedicate 50 hours of work tearing out the fresh water tank. It looks like a huge job, I don't have the time right now.
Increasing the size of the cut out in the subfloor would be a challenge. The frame rail is right next to this array of tubes and fittings. A cross member is just in front. And the "microwave" pantry cabinet wall is right behind it . I'd have to remove all that cabinet to get some access to saw a bigger hole. Believe me, I tried to find a way to gain access.
The push to connect style connectors are about 2" long. I don' have the room in there to work them. I hear ya about those three drain valves. They are not needed for me as I use air pressure to blow the water out, and then RV antifreeze pumped in every line.
I'll plan on replumbing from the 12v
pump to the galley faucet and then under the floor to the street side bath when I drop the fresh water tank.
Here is a photo of the leaking tee devil. I think it may have rubbed itself through against the frame rail. But I can't see it.
And a photo of the new PEX I installed some years ago from city water inlet, water heater, and bath fixtures; toilet, sink and shower. Note the push to connect PEX to poly fittings I used to, well, connect up to the old plumbing.
I'll let you know if I can slow down this fresh water leak...