There are combined Co and LP detectors, so watch out what you have.
The propane detector should be mounted at floor level because propane is heavier than air, but the CO detector which you find as well as LP detectors in MHs and TCs (the truck's exhaust might get up into the TC)
So where do you mount the combo version? Not a clue :(
Meanwhile, since the trailer has no engine why should you have any sort of CO detector and with no LP either, why have one for that?
Possible unlikely scenario, is if you are too lazy to un-hitch, and you leave the truck running so its exhaust gets in the trailer?