A little more about the grey tank responding to some replies here. The story is actually a little longer than I originally posted. Hopefully some will like the additional detail.
Because my previous motorhome reported 1/3 grey full after only a little water was used, we ran a test to know exactly how many gallons triggered each light. So, with the new motorhome, ran the same test - just to know - no witch hunt. This is when I discovered the lower usable capacity.
The dealer had to repeat the test because FR would not believe my test. They got only a 1 gallon higher result, still well below 41 gallons. While doing their test, water leaked into the compartment behind the passenger rear wheels (what we call "Door #4). They immediately said the grey tank is leaking before checking any fittings and ordered a replacement. I got to explicitly measure that tank for it's dimensions. The tank was stamped 41 gallons and my calculations with assumptions on material thickness came in at 40.7 gallons.
I could not repeat the leak and refused to have my coach torn apart for a grey tank replacement without a definitive diagnosis OR to just get the same capacity back. So, my conclusion was the drain pipe insertion distance, which, one cannot examine without taking the pipes out of the tank. Maybe some day, especially if I get another leak, I'll take apart the bathroom/bedroom wall and disassemble/check the pipes (seal integrity and depth).
The grey tank capacity does not include anything else (e.g. water heater). As one cannot get their hands inside the tank, I can only imagine that the seal of a pipe to the tank is drill a hole, stick in the pipe, and use glue. If there is no specification of insertion distance (or it's ignored), the tank usable capacity will vary.
2015 FR3 30DS
2010 Ford Escape Toad