I cannot speak directly about the model 400, do know on the model 320 nüCamp installed the largest possible tanks for the available space. Expect it is far easier (and far less $$) to learn how to "live within your tank capacity" then undertake a DIY change.
Here's an example 320 tank placement:
'18 320 S, pitched axle, 3020HE; PNW based TV: '17 Colorado V6 Z71 4x4, Tow Package, GM Brake Controller Adventures: 54Nights: 341 Towing Miles 43,780
Basically no. The Black tank is part of the washroom, and is the raised area under the toilet. The fresh and gray tanks occupy all the remaining space under the trailer in front snd behind the axle. If you need larger tank capacity, get one or two of the aftermarket external storage tanks youmcsn drain your gray and black tanks into. cheers
2018 TaB400 Custom Boondock, Jeep Gladiator truck, Northern California Coast.
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Here's an example 320 tank placement:
TV: '17 Colorado V6 Z71 4x4, Tow Package, GM Brake Controller
Adventures: 54 Nights: 341 Towing Miles 43,780
cheers