I would get one like this. A combination 12 volt hard wired unit.
<a href="http://www.amazon.com/gp/product/B003ZJLD3G/ref=as_li_qf_sp_asin_tl?ie=UTF8&camp=1789&creative =9325&creativeASIN=B003ZJLD3G&linkCode=as2&tag=aff iliatechie-20">RV Motorhome and Trailer 12V CO Carbon Monoxide & Propane Detector Alarm</a><img src="http://www.assoc-amazon.com/e/ir?t=affiliatechie-20&l=as2&o=1&a=B003ZJLD3G" width="1" height="1" border="0" alt="" style="border:none !important; margin:0px !important;" />