Haven't seen any replies to where it's located so will mention the two I know.
On our 2003 3295RK it was under the bathroom sink.
In our 2007 3400RL it's behind the pantry wall, between pantry and shower. In the pantry is an access panel stuck on with two-sided tape. However, the hole and panel are in the wrong place. I could see the valve through that hole but had to enlarge the hole to get to the valve.
Here is what it looks like.
This one is in the 2007 3400RL
