Have you considered moving the water tank forward under the Vberth instead of under the settee? 

I've been thinking if/when I replace my tank I may consider putting it up forward.  Having it forward would help eliminate the starboard list when the tank is full and might offset some of the stern heavy tendency the W28's have.

I looked at a SeaLand rectangular tank that has 35gal capacity with a V  taper on the bottom. #35HTS-HL   It looked like it would fit nicely in the center of the Vberth storage. 

They also had a 52 gal V shaped tank that might fit. in that same space.   #MSD316405200

SeaLand lists these tanks as Holding Tanks, but they could also be used for water.

I have seen ads for companies that will make up custom Poly tanks.