Sure looks like all the black wires were supposed to be connected using a wire nut, notice how the wires are "twisted". The national electrical code indicates that the black wire is the "hot" wire; i.e. the power source. To control a light the black wire should go from the power source to the switch than on to the light with the white wire being the neutral wire. Flipping the switch connects/disconnects the power to the light. Since the light is staying on this would indicate that the switch is being bypassed somewhere in the circuit. If you disconnect one of the black wires from the light switch the light should go off, if it does not than the light is getting power from somewhere else.
A second scenario could be that the 3rd wire is supposed to control another light. Is there a light that is not working; if so, than that is what the the 3rd wire is supposed to power. Both lights being controlled from one switch. From your description I don't think this is the issue.
Now since the light does not go off when the switch for it is turned off I suspect it is getting power from a second switch so the light can be controlled from two locations - however, if it is connected to a second switch than there is a major wiring problem. Sounds to me like a previous owner did some wiring which they did not understand how to do correctly. Controlling a light from two different locations requires switches designed for that purpose. When you removed the switch in the above step were there only two wire connections on the switch, not counting the green ground connection if there was one? If so than the wrong switches were used essentially creating a "short". When controlling a light from two locations each switch will have three connection points, again not counting the ground connection, to allow either switch to turn the light on/off. The switches are interconnected using the third connection point. This kind of problem is best corrected by a professional RV electrician if you are not familiar with electrical wiring.
Seeking professional help when electricity or gas is involved is cheap insurance. Hope this helps you in solving the problem.
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