Your water heater plumbing may be one of 3 different set-up.
1 valve only with 2 check valves
2 valves with 1 check valve
3 valves with no check valves
The basic plumbing set up for all our water heaters in our campers is an H pattern
Think of the left and right straight lines on the "H" has the water in (cold line), and the the other side of the "H" as the water out (hot line).
Between the two, the horizontal line on the "H" is a transfer line that bypasses the water heater when both the "in" and the "out" line are shut off. The middle part of the "H" allows the water to continue flowing without going into the water heater (thus - by passed).
Here is a 3 valve system.
Here is a 2 valve system
Here is a 1 valve system
If yours is a 2 or 1 valve system, then the other valves are in-line check valves. That means water can flow one direction, but not the opposite way. There is a built in valve that detects when the water pressure reverses and the valve closes preventing water from flowing backward in to the water line.
Depending upon your camper's configuration, you no doubt have a second valve. The one turned off is the middle part of the H. In your camper, I have no clue where it would be located. But it should be close to the water heater, and very closely located near the valve you did find.
On EDIT:
Since I started putting this dissertation together, I see more postings occurred on this topic. Yours has a wet bay, (5th wheel????). Agree with the above poster, it looks like you have one valve on winterizing.