My Model B 29' Class C Winnie has a large (1 1/2" or so) valve in the compartment containing the internal water pump. It has a yellow handle, marded WATTS, and appears to be in a line between the main fresh water tank and a smaller, lower tank in the compartment. It is labeled simply ON and OFF. My manual does not make reference to this valve. Can someone fill me in on it's purpose? Thanks.