Firse and most importantly, gas detectors have a lifespan. Usually 5 years is the maximum, due to the material in the detector degrading over time. If your gas detector is over 5 years old, or you don't know how old it is, it's time to replace it. If you have built in's, you can usually get the same style as a replacement, but be aware they may be wired to 12VDC and only work when the house battery is on. This does you no good if you shut the 120 VAV and house battery off when not in the RV.
Remember LP and NG are heavier than air, do not mount the detector above the counter surface. CO is about the same as air and will disperse throughout the space, so a combo smoke and CO detector is fine to mount on the ceiling.
First Alert and GasKnight are reputable brands. First Alert are battery powered, GasKnight is plug in.
https://www.firstalertstore.com/store/products/gco1cn-combination-explosive-gas-and-carbon-monoxide-alarm.htm
https://gasknight.com/