Here is a picture of the fresh water tank and the plumbing that should be underneath the passenger bed. When you lift the mattress you'll see a plywood panel with some screws. Remove the screws and lift the plywood and you'll see what the picture shows.
The water pump is located off to the left just beyond the picture. The big gray thing at the top of the picture is the fresh water tank. Look at the first blue water line coming out of the tank (the blue line on the far left). That is the cold water supply line that goes to the water pump. Notice there is a valve on the line just before it goes to the T-fitting. Your valve should be in the same position as the picture shows -- the handle should be parallel to the water line (open). If it is perpendicular to the line the valve is closed. Be sure this valve is open.
Of course, verify there is water in the tank.
I compiled a Vegas/Axis Owners' Manual that supplements the information provided by Thor. I explain how all the systems work and there are about 70 pictures in the manual (the picture above is one of them). I included the link to my manual that you might find useful. You can download it as a MS Word or PDF file.