Yes and Yes, though it's a bit of a pain. The check valve is on the back of the water heater as you've been told. To access mine (same floorplan as yours, 1 year older) I had to get underneath the coach to get up behind the water heater bay. There was a cutout that allowed access to the back of the water heater. Tight fit but it can be done, though the next time I'll hire a younger guy to do it or go to a shop where they have a lift or pit.
Ed & Joie
2004 Vectra 40KD
Freightliner Chassis and Cummins ISC