karebear - The correct term for the "fish eye" is the fresnel lens as CharlesOK points out. I refer to it as a "fish eye" as that is what others have called it and it does look a little like that until you attach it to the inside of your window with a little water or windex. The rear window being tinted or not should not affect the way the lens works. I use it extensively - not so much for detail, but knowledge that something is back there (large or small). I can back my trailer with it - no problem (worth the money).
We do haul our motorcycle with us on a homemade "low to the ground" trailer. It has a drop axle with 13" car rims making the bike only about 10" off the ground. I also have a rear carrier where I roll on a Honda 150 Elite Scooter when not towing the road bike. And....yes price of gasoline makes a difference when the bike gets 55 mpg and the scooter gets 75 mpg.
CharlesOK - A wise trailer builder once told me to make sure the distance from the trailer axle to the ball coupler is twice the distance between the wheels. Makes for better towing. I also know that you need to put the bike far enough forward to have about 10% of the weight of the trailer and bike on the coupler. So....if it does not pull nicely..check these two things.