The easy part is getting the RG cable to roof area... I have run several different Rg cables to the roof area for Ham radio and Police Scanner antennas. There are multiple ways getting to the roof.
The hard part will be to use the same Roof AM/FM Radio antenna as it is most likely having the RG cable come through the roof directly under the antenna assy...
I would most definitely pull the RG cable from the other end of the antenna on the roof area and see if the existing RG cable is good or bad. Problem could in the antenna assy. I really doubt you will find a "CHRUSHED RG CABLE"... Maybe cut into or bad connectors etc (especially the connector on the roof)...
If your antenna mount has the RG cable just laying on the roof and plugs into the antenna assy then you should not have any problem with just running a new RG cable to it...
There are several marine stubby type AM/FM antennas available that would work great on your AM/FM Radio.
If the coat hanger is working great you might want to consider an indoor AM/FM antenna using twin lead cable. This might be very easy to blend in with your inside decor... Most trailers have high ceiling mounted storage cabinets... All of your wire for the antenna could be very easy glued inside the long cabinets...
Roy Ken