The valve under your sink, like Mike said, is your low point drain valve and there are two of them, one for hot and one for cold. These are opened only in the fall when you winterize your trailer and need to make sure all water is out. (There are several threads on here about winterizing already, so I won't go into that any further. They are recommended reading for you.) Anyway, for camping season, the low point drain valves need to be fully closed so that no water comes out of the drains which are visible under the trailer.
Here's the backside of the hot water heater from my '99. It has two bypass valves, top and bottom, right near the HOT and COLD labels. On newer units, there is only one valve. Either way, you should be able to feel around and locate the valve(s) on your unit.
If you have a two valve system, one (marked COLD) has to be open to allow water to flow into the heater, and the other (marked HOT) has to be open to allow water to flow from the heater to the hot water taps in the sinks and shower.
If you have a one valve system, flip it one way to bypass the heater for winterizing. Flip it the other way to route water into the heater for camping season.
It may be helpful for you to document the valve positions for both settings so that you can easily look it up every fall and spring. I keep my notes in the binder with the rest of the trailer manuals and such.
'12 F250 4x4 Super Duty PowerStroke 6.7 diesel
2011 to present: '11 Cougar 326MKS
1999 to 2011: '99 Sunline T-2453
Amateur Radio kd2iat monitoring 146.52