I have a 2004 winnebago Journey 36G. I have replaced the check valve behind the water hearer a couple of time. Today while unwinterizing all was going well after flushing the water heater and lines....
Put the back slide out, take the drawers out of the "chest", and look down into the hole in the floor. You should be able to see the check valve. Getting to it may be another problem. Long, strong arms is necessary to remove and replace it. If you go behind the metal panel a "basin wrench" is helpful. Good Luck, Indiana Journey