Combo Propane/CO
Propane is a heavier than air gas (Natural gas is lighter than air by the way)
So propane detectors are normally mounted near the floor
CO,, Well that's a bit more complex but it often concentrates at the ceiling so the CO detector should be within 4 Inches of same.....
Likely less expensive to get one of each.. With propane shut off there is no danger.
Kidde CO detectors.....
My RV came with an Atwood unit, which ultimately died of old age,,, I got a Kidde to replace it... here is why
First... half the pages in the owner's manual said kidde not Atwood, clearly they are the same product.
Second. The only difference was the name, a very slight color difference (likely due to age) and on the Atwood unit the battery cover (it runs on 3 AA batteries) had a sticker/decal with the different alarm status indications explained, Not so on the Kidde.... (I simply exchanged that removable cover)
Finally.. Fits the same mounting bracket,, Exactly the same. No problem.
Id' split your CO and Propane and mount the CO high.
Now this brings up a question.. Why do they make Combo Detectors?
For use in a Sticks and Bricks with Natural gas of course.