We similarly have been frustrated only put 11-12 gal in with the low fuel warning on in our GHH limited. I did get my dealer to start a TAS case and try and remedy the situation. They apparently only actually did the second time I brought it in... They then told me they recalibrated the sending unit and thought it had 6.2 gal when I left. They asked me to fill it up right away and send them a video of it. They expected it to take 11 gal, it tool 9.7. I was suprised the next fillup around 15 dte took 13.5 gal when the pump shut off. Some movement in the right direction.
I took it back a third time and Toyota directed them to replace the sending unit. Howerer they reported the new and original units both tested and reported the same. So, the note says they left the original installed. When I picked it up they thought the tank was too small. The tech thought it had 1/3 tank (~5.5 gal) and they filled 8.3 gal testing themselves. A couple days later they emailed me that the sending unit doesn't read the first 2.5 gal reserve. So, they now think it adds up.
Unfortunately their math is wrong. If the sending unit only measures 2.5gal to 17.2gal then 1/3 tank is 14.7/3 = 4.9 gal (not 5.5) + 2.5gal reserve. Leaving their total only 15.7 (4.9 + 2.5 + 8.3). It seems like their logic is flawed that they don't actually know the amount of fuel in the tank. They are basing it off of the sending unit, which is in question.
So, I decided to try and test the capacity myself by siphoning it out. It turns out you can siphon the gas out of the tank with a 1/4" pex line. So, I drained the tank when it was down to 1/3 again. (FYI I don't recommend the 1/4" is very slow. It took nearly 3h). I pulled 7.25 gal, but I don't think it drained fully, as it still started and ran fine for a few minutes after (I tried parking on an incline to get more out). Interestingly it continued to state 1/3 tank while moved it around empty. Not wanting to run completely dry, I put 2.5 gal back in, drove to the nearest station (2.1 miles away). It listed a DTE of 15 miles when it started and 13 at the pump. The low fuel light was on the whole way and it accepted 14.01 gallons when the pump shut off.
So, I confirmed the tank holds at least 16.5 gal. I think it likely had the remaining 0.7 below where my siphon could reach, or in a depressed/compartment near the pump pickup. Especially with the earlier report of 18 gal when running completely dry, and the DTE of 15 after adding 2.5 gal. If there is a 2.5 gal reserve the sending unit doesn't know about, it makes sense that it registered that it still had ~0.5gal above that reserve.
There is the chance that my tank is slightly smaller and others larger. Doing some reading it on various sources, I found its common for vehicles to be +/- 3% of stated capacity due to manufacturing tolerances. That could swing 17.2 +/- 0.516 gal. So, it's possible my capacity is slightly diminished, but not the 1.5 gal based on their math (17.2 - 15.7). Planning to follow-up with the dealer after our next tank/fillup.
