On our 370 there are two black and two grey water tanks. I don't believe any to be 41 gallons. Combined they most likely equal that 41 gallons for each color so to speak. As for adding anything I don't see any place where I could add or change.
The blue tote-alongs may be the only option if you want additional greay water capacity but, I don't think I'd want to go that way for additional black water use. Transferring waste from the holding tank to that would require watching to avoid overflow.
Not a pretty sight!!!!