On my 2022 29S, the second grey tank is only for the outside kitchen sink and is pretty small...I saw the tank heater sticker on it but no capacity plate.
My guess is pretty small maybe 8-15 gallons or so. It has it's own dump valve and outlet too behind the back wheels.
Hopefully it's pertinent info though a different model year. The brochure for the 2022 model year doesn't mention the second tank either.
Retired USCG Helo Pilot, Retired USCG Lic. Captain
2022 Entegra Vision 29s, 2021 Ford Ranger toad
Ft Pierce, Fl