Mag mounts are intended for use on the roof, and it must be a STEEL roof, (Aluminum and plastic don't work well here) or at least steal taped/plated for the mount.
I would recommend a mirror mount or drill holes,
Next, Unless you are like me (WA8YXM) be aware that many states regulate scanners in cars and trucks, including motor homes Having one in your motor vehicle can get you in trouble.
Michigan (Where I live) prohibits them unless you are a bonifide Law Enforcement Officer (I think Firefighters are also exempted) have a permit from the director of the state police (A good friend of mine) or are a bonifide amatuer radio operator of Technician, General, Advanced or Extra Class (I'm a Tec) Most states exempt ham radio operators
The law actually says 'Radios capable of receiving frequencies assigned by the Federal Communications Comission for use by Police", it does not mention "Scanners" per se