"People sleep in motorhomes or cars all the time."
Not to disagree, but IIRC it was all the way back in the 70's or 80's that the US government
passed a change for passenger cars. If the ignition is on, even in the "accessory" position, the ventilation fan MUST run at low speed. So, you park your warm toasty car on a winter day, leave the radio
on for a minute to listen to a song, and there's suddenly an icy breeze in the car. Or worse, a hot muggy one ruining your cool cabin
in the summer.
Apparently it isn't all paranoia.
Regarding the OP's question, like they say about the price
of a yacht, if you have to ask, you can't afford one. You need to monitor oxygen levels, carbon monoxide levels, and carbon dioxide levels. Your body normally does this and unless you've got carbon monoxide problems ("the silent killer" prevented by a $30 detector) your body should wake you with a nasty headache or other way of telling you to get some air.
Or, you do like the coal miners and sewer workers and commercial storage
tank inspectors, and buy a "mixed gas" safety
monitor. You might get one for a grand.
Might not be time to move into a boat until after you're more familiar with the realities of it all.