Probably the best place to get a propane detector is at the RV places, they usually have several types. I think they'll also detect gasoline vapors. They are VERY sensitive.
It does seek the lowest level but where it ends up is going to depend on the structure of the boat, breeze if any etc. You might want to install more than one. Mine is located in the cabinet under the stove, which also houses the water heater. I felt if I had a leak that's the FIRST place the gas would collect. However such things as roach spray will set it off. Annoying.
I'd be reluctant to install a fan, it could be a source of ignition.
She's a tired old barge but she's paid for! http://s71.photobucket.com/alb...p;current=ef324993.pbw