With most vans it will be immediately after the mains water inlet.
Try to follow the inlet pipe from where the hose clips on, along the pipe. In most cases it will be near the inlet and secured to the chassis or to the floor with a pipe clamp or it will rely on the pipe clamps that secure the pipe that connects to the pressure regulator.
The pressure regulator is generally a round brass unit approx 100 to 150 mm long and approx 30 to 40 mm round.
I hope this helps you to find it