fuse or circuit ?

Fuse. If it's blown, you should see a red light next to the blown fuse. If not, the detector itself may have failed. If it's the original detector that came with the Pod, it needs to be replaced anyway because they only have a 5-year service life.

If the LP/CO detector were out of date, it would be flashing lights and beeping. It is quite annoying. I had to replace mine. Some people have just taken it off and cut the wire and then put it back. To me, it does sound like there might be a wiring issue. However, it could be that the detector is broken in addition to being out of date. If yours is like my old one, it was surface mount. I recommend replacing it with a flush mount model. It costs a bit more, but not kicking it almost every time I walk past made the cost worthwhile.

They need power to beep. I don't know how long the continue to flash and beep when they are out of date. It was too annoying and we had trips planned to try it. I could have saved it and connected it to a battery to see how long, but I did not think of it. Besides, it would not have gone over well with the wife.

I have a 2019 179 and my CO detector lost it's life this spring on a trip we were on to NM for 6 weeks and it gave us a audible notification so we had to replace it.  The detector is hard wired into the trailer's 12v electrical system.  Meaning that the detector had to be unscrewed from the wall and the wiring exposed and disconnected.  Then the new one installed.  On my trailer the circuit that the CO detector is wired into is circuit #3 which also has the wiring for water heater control circuit, the LED level indictors for the water tanks, and the water pump.  Note that circuit is always live if the trailer's battery(s) are connected with power.  If the fuse is disconnected or is blown the circuit is not powered. 

Yes, the floor has a thin layer of plywood over a foam core. Note that the trailer walls rest on top of this plywood/foam sandwich so if the leak was near the outer walls you are pretty likely to get sagging.

Lots of owners have reported that problem (with or without water damage), and FR now makes outriggers you can install to help with that. You can use the search function to find out more.