Since Grand Design started putting OD water heaters in some of their units (the Solitudes and Momentum fifth wheels still have 12-gallon standard WHs), there have been many, many posts about problems with them over on the (older) GD forum. There are the problems that dougrainer mentioned plus: 1) without the option to use electric, they go through propane rapidly, and 2) the Furrion units are not insulated and are notorious for freezing and cracking the coils when the outside temps drop or the units are not properly winterized. I would never install one.
Rob