Not inexpensive to purchase from dealer (Lippert) but they are available. As I recall, they have a two valve and up available. Now, that said, if you always want to have the main slide open regardless, then you should be able to go to a good 'hydraulics' shop and have them make a single valve assembly and the hose. Put it so that you can shut off the fluid going to the bedroom slide while still leaving fluid going to the main. My neighbor with a different brand put a valve on the main slide -- figured he wanted to be able to open the bedroom slide when making short stops when traveling but the main could stay shut. Cost him a little over 200 for the valve and hoses he needed -- had them made at a farm tractor repair facility.
(Had to edit that last sentence - as originally typed, sounded like he was driving down the road with the Bedroom slide open -- definitely not the case
.)