Suffocate while Living aboard a coverd boat

[Eastward ho 24]

Is there a monitor that's affordable(cheap) that monitors oxygen levels? My boats coverd on the hard it's got a plastic dome cover on it. The covers got a door and 2 4"x4" vents. I plan on sleeping on the boat while it's covered . I've done it before but I feel claustrophobic. The boats on the hard is there a serious threat of the o2 filling up the boat and killing me? Is there a monitor I can only seem to find a propane alarm. I'd sleep better knowing I'm not gonna suffocate in my sleep. Should I be concerned