We've monitored a lot of motorhome user groups for various discussions, but have yet to hear of serious theft from the inside of a motorhome, although we're sure it's happened many times over the years. Thus, while we don't feel the need for an active alarm system (motion sensors, shock sensors, IR sensors, pagers, connection to security firms, etc.), it would seem prudent to take some measures, especially when a motorhome is left unattended for periods of time in remote areas.
The best result you can get with security measures is deterrence. If you can get a thief to stop considering entering your vehicle, you avoid all of the nasty consequences of an actual break-in. All you need to do is to make your RV look less desirable than something else. Like the joke: You don't have to outrun a bear, you just need to outrun your tentmate.
Thus, the following describes two ways to hook up a simple flashing red light to make it appear as if you've got a full-blown alarm system installed, each approach costing maybe $5. The placement of the LED is left to you, but we feel it's important to mount it so the LED can be easily seen by someone at a distance without being obscured by drapes or screens.
The idea is that you connect the positive side of an LED to a constant source of 12v, and the negative side to the Ignition circuit. The Ignition circuit is normally at 0v until you turn the key on, and then it's 12v. Thus, when the ignition is off, the LED sees 12 volts across it and flashes, but when the ignition is on, the LED turns off because there is no voltage difference across it to power it. This allows you to travel without having the light confuse people, but when you stop the light will flash.
Flashing red light, 3v, 35 ma (Radio Shack p/n 276-312)
1/8 watt (or larger) resistor, 400 - 2k ohms
Suitable LED mount
On the Discovery, you can find the constant 12v and the Ignition power under the console "snack tray" pull-out. The constant 12v comes from the Remote Entry Module's red wire, and the ignition 12v comes from the Radio Relay Module's leftmost white wire connection. Picture Tap into these wires for the connections. Add inline fuses if you are at all concerned about a possible short somewhere, or use the smallest gauge wire possible to act as fusible links. The LED suggested above is designed for 3v, so the dropping resistor needs to be wired in series with the LED to allow it to connect to 12v. The flash rate for the LED listed above is about once per second. If you buy a flashing LED rated at 12v you would not need the dropping resistor. Optionally, you can add a switch anywhere in the circuit to disable the flashing (and average of about 5 ma drain) when it is not needed or would be confusing.
The Radio Shack part number 276-299 is a two AA-cell battery pack with a small LED carrier board attached with a short pair of wires. The battery pack has a switch which allows you to turn off the one-second blinking red LED to save power, but the LED will work continuously for about six months on two AA batteries. The LED board has an adhesive circle allowing for simple attachment with a through-hole mount. Instead, we mounted the board inside a small black plastic box and glued the box directly to the battery case, making for a single assembly. This assembly is Velcroed against the lower edge of the passenger side window (using an existing window screw to connect the Velcro strip) so the LED can be easily seen by anyone outside the motorhome.Picture