Probably the battery, but how old is the trailer? The various detectors and alarms in trailers have a finite life span and have to be replaced every so often. Our old hybrid was 9 years old when we traded it in, and was already on its second CO and propane detectors. Each one had a "replace by..." date on the back. The devices themselves alerted me to this when they would beep about once every 30 seconds or so.
Geoff & Jill
& Sierra, the little white monster
2013 Ford F-150 XTR SC Ecoboost
2015 Jayco Jay Flight 24FBS