This happens on my hurricane from time to time. I disconnect from shore power and put the coach in store mode. This will turn off the light and when I put it back in use the light will be controlled bu the switch. Not sure of the root cause.
Well, the easy first guess would be your exterior door light switch has failed shorted (on) and needs to be replaced. You could verify with a voltmeter, but in effect since your outside light stays on regardless of switch position, it is acting like a "test light" for you already. The other possibility is that the wires on the back of the light switch itself are touching each other. Remove the switch (or panel its mounted on) and see if there is something obviously shorting before ordering a new switch if not under warranty still. Call Thor for a part number and order through their retailer.