You probably already know this, but you can actually measure the draw with an ordinary multimeter:
1. Make sure everything in the RV is off.
2. Disconnect the battery, using the battery disconnect switch.
3. Put the red (positive) multimeter plug into the 10 amp socket on the front of the multimeter.
4. Unhook the negative terminal of the battery but not the red positive one. Using an alligator clip or tape, clip the multimeter's red lead to the negative cable (the one that would've been connected to the battery).
5. Clip the black lead to the screw post of the negative terminal on the battery. This is hooking up the multimeter "in series" – i.e., the current has to flow from the negative battery cable through the multimeter to the negative terminal.
6. Turn the multimeter on to the 10 amp setting. It should read "zero."
7. Reconnect the battery, using the battery disconnect switch. You should get a fairly low reading – around 0.2 -- the background draw of the carbon monoxide monitor, the smoke detector, and the propane detector.
8. Turn on the device you are trying to measure. So, for example, if the background number is 0.10 amps, and the meter jumps to 0.30, you know that whatever you are testing draws 0.20 amps.
9. Disconnect the battery. Reconnect negative cable to negative screw post. Reconnect the battery. Restore the red plug to the V Omega mA socket on the multimeter.