I own a 2017 Chateau 31W. The water pump is under the bed next to the fresh water tank. The compartment you mentioned is the hydraulic system for the leveling jacks, not the slide out.
I've attached a PDF for the water pump, and the water system schematic. I tried to attach the full owners manual, however the file was too large to post here.
Here is a link to the Thor owners' resource web page: You'll need to create an account with your coach information. It has a wealth of information about your coach to include all (or most) manuals for the on-board systems. Also a copy of the coach owners manual.