Makes more sense than 100 gal fresh and twin 45's for black and grey, but that's the end of it.
Many times the manufacturers just bought what was available as far as tanks ( size wise, black/grey ) and fitted them in rather that custom ordering. Tank location, well my first 2 , 5ers were as you describe, tank under the dinette , only when I got my third, a 2000, was the tank a good size and location; slung between the frame rails , directly above both axles; problem with that was , fresh tank was not heated so freeze up was a consideration. Above floor location was primarily to avoid freeze up so they fit in the tank size they could. If you have the weight carrying capacity you can add a larger tank, keep in mind side to side balance and the pin weight or lack of that 850lbs of water will produce.
If your out in cold areas then heat the tank, somehow.