Replacing black water tank with freshwater tank.

You may need a new pump if your existing FW tank is not below the floor at roughly the same level as the existing black tank. Also if you replace with a FW tank that sits lower then the pump pickup, might be a problem.

Resale value will likely be very poor without a black tank. If you can swap in a FW tank without making heavy modifications so that the black could go back in.. that would be good I think.