Hmm, interesting.
Many non-Revel 2019+ Sprinter friends have gone with larger fuel tanks, but their vans weren't factory optioned with the J51 coding (proven ahead of time by the VDoc from a Mercedes parts manager--a friend of mine so I know he's not lying, especially when his dealership isn't a Sprinter dealership so he has no dog in the fight), so their fuel level didn't read full with a larger fuel tank. Once their van had the J51 added at a Sprinter dealership, then their fuel gauge read full with the larger tank. However, I don't know that they asked for proof of the VDoc after J51 was added. Asking now...
Yep, at least one friend's 2019 2500 showed on the VDoc that it didn't have J51, so he went to a Sprinter dealership, had J51 added for the large tank, and now the VDoc is showing J51 (as proven by my friend the Mercedes parts manager, who is at a non-Sprinter dealership hours away).
Bigger question for Mercedes USA is why didn't all vans, 906 and 907 chassis, come with J51 from the get go? All of these vans have fuel level sensors in the fuel tank, so why would Mercedes option some with J51 in order for the fuel level to read off the level sensor, and some vans not have J51 and thus calculate fuel level off of the trip computer? Was Mercedes just trying to get extra money from the non-J51 purchasers?