Most older trailers did not come with propane
alarms (also called detectors) but you can certainly add one. The replacement one for our trailer cost $56.
came with a Safe T Alert, here is their web site,
This seems to be a good brand, the local RV dealer carries the same brand.
There's surface mount, which attaches to the wall, and flush mount, meaning you cut a hole in the wall to mount it. Propane is heavier than air so it must be mounted low in the trailer. Ours is on the bench seat in the dinette, under the bed when the bed is set up.
LP detector is a must, a CO detector, like you have in your home would especially be important in an older trailer that might have an older furnace
which could have a crack in the heat exchanger allowing exhaust gases to infiltrate into the trailer. I believe most trailers do not have CO detectors, but others on the forum might have different opinions on this. A CO detector is usually mounted higher. SafeT Alert carries both and has a dual alarm that includes both. It is a matter of personal preference and what you feel comfortable with, whether you have good ventilation, the type of furnace
you have, is it direct vented, etc.
You can probably go to the local RV dealer/service center and have them install which ever alarm you decide on for a reasonable cost.