The plumbing inside a mobile home is pretty much the same is it is for any site-built home. Each fixture has (or should have) individual shut-off valves. Shut-off valves at the toilet, under the sinks and even at the water heater can be used to turn off the water supply for that particular fixture.
But shutting off the main water supply in a mobile home can be a little more difficult than in a site-built house because mobile homes are raised up off the ground and the shut-off valves often come in from under the house, where they are protected.
The location of the main shut-off can vary, so it is just a matter of checking various locations to find the valve.
Where can you find the main water supply?
Look for the main shut-off valve where the water main ties into the mobile home water line. There should be a gate valve or ball valve at this location. Getting to it is another story.
If you are lucky there is a section of easily removable siding nearby that you can just unscrew or slide out of the way to get to the shut-off valve. Sometimes there may be a vent nearby that you can remove to reach the shut-off valve. Another way is to just climb under the mobile home to find the shut-off valve and then turn it off.