Are you talking about a leak detector (for unburned LPG gas) or a detector for carbon monoxide?
The Mitre 10 Quell CO detector ($49) seems a lot cheaper than the ones offered in various RV accessory shops:
http://www.rvsupplies.co.nz/products/se ... m=detector
https://www.rvworldstore.co.nz/catalogs ... q=detector
But it has a CE EN50291 certification mark on it so it should be fine:
http://www.carbonmonoxidekills.com/carb ... -european/