After 28 years in the radio business, Bad grounding for the antenna and radio chassis is the most common problem with NO AM reception and weak FM signals. If your radio will not receive AM stations unless you are setting directly on top of the transmitter and very few FM signals when there are 25-30 FM radio stations with-in 30-40 miles (FM RANGE). Ground the radio chassis and the antenna mount. The antenna ground shield is isolated from the chassis. When the radios are installed in a RV at the factory, there is no metal in the roof to ground the antenna to.
It takes a pretty smart "BUBBA" at the factory (installer) to do a correct installation including a GOOD GROUND for the antenna which sometimes will require a good ground ALL THE WAY TO THE RV CHASSIE OR FRAME. The ground for the 12 VOLT system is not EARTH GROUND!
28 YEARS OF RADIO WORK