I use Fresh Cab when we leave our trailer in the woods for several months in the summer. Have not had any mice problems. Of course, we always vacumme well and make sure there is nothing left in the cupboards that the mice might be attracted to.
The Fresh Cab has, to me, a pleasent flowery smell and looks like it is made of dried flowers or herbs. I have left our trailer up there for about 5 weeks without being used and the Fresh Cab smell had mostly disappeared when we returned. No traces of any mice droppings.
I would, if possible, replace it monthly. You get four bags in a box and used three bags at once in our 30ft trailer.
Don't know about using them under the propane cover. We have the hard plastic cover and have never had any problem with anything getting under there. I suspect that the smell of the Fresh Cab would dissipate quite rapidly there and the effeciveness would disappear.
Barney