I looked at ours and it has a sticker on it that says RED-ALARM GREEN-GOOD YELLOW-REPLACE. The green light is on showing that it is working. Ours is mounted in the middle of the back wall under the rear window along the floor. I am surprised it is mounted so far away from anything that uses propane like the furnace, water heater or fridge that are all in the middle of the 3370RL. If yours doesn't have a light on it showing the condition of it maybe it is time for an upgrade. John do you know how much gas it takes to set these detectors off? Does your fire department have any gas they use to test them with? By the way there was a house fire about a half block up the street from us the other day. The guy said all he was doing was microwaving a pizza and the next thing he knew the whole room was on fire. The fire department put it out but it caused extensive damage. Looks like they are rebuilding the house now.
Interesting questions. First off, no we do not carry test gas for residential or RV detectors. We have calibration gas for our detectors, but it requires adapters to attach it to them. If someone's CO detector, for example, is going off in the house, we take an initial reading at the entry point. If the meter is starts to rise over 35 ppm, we mask up and proceed to survey the home to find the source (usually either the gas stove or the furnace/water heater). If they complain of gas odor, then the same procedure with the meter, watching the %LEL reading. At 10% LEL, we beat feet for the door. Changes in the oxygen level readings are also an indicator of a problem. I have a Powerpoint slide about LEL/UEL I use for training, but it won't paste here. Our meters start displaying at 0.1% LEL for methane (the calibration gas).
RV gas detectors are set (from the literature I found) to sound at 25% of the LEL (lower explosive limit) for propane, which is 2.2% by volume in air. So, that means if the concentration of propane in the rig (at the sensor) is 0.5%, the alarm will go off. This does not mean imminent explosion, but it's time to take action (evacuate, call 911, turn off gas if you can safely, ventilate). On your rig, turning off the gas at the tanks should be enough to secure the situation if done quickly. Some of the literature I looked at says the (brand) of RV detector is set to go off at 25% of the LEL for both propane and methane. I question that since the LEL for methane is twice that of propane (5%). The sensor can only be calibrated to a single gas and the user needs to know which. Encountering any other gas requires a correction (K-factor) applied to the meter reading (if you know what the gas is).
As for microwaving the pizza, unless he had a major gas leak in the kitchen, there are only two ways I know of to start a fire with one. Either an electrical short or putting metal inside the microwave. I know, nobody ever does that...
http://www.es-web.com/PDF/install3.pdf
Be mindful of your safety, but remember that these detectors (CO, LP, smoke) can malfunction, even when relatively new. Normally when they do, they sound an alarm as a fail-safe so you do something about it. And, as we tell our residents,
NEVER be embarrassed to call the Fire Department because your alarm has sounded. I'd rather pull the old batteries out for them than carry their corpse to the curb.
In regards to the mercaptan odorant, the normal detection threshold is 0.002 ppm. It's used because the normal human nose will detect it long before the gas concentration is anywhere near a problem level. Natural gas and propane, themselves, are odorless.
This has been gas monitoring in a nut-shell. It is by no means all-inclusive or meant to imply I am an expert in this.