I have a 2012 Damon Challenger. I have a problem with dash radio and outside radio not receiving and signal. When you first start the coach the radio works fine. But, after about 10 mins signal go...
From your description I would not suspect the Antenna. Antenna's usually either work, or not work. Not start and then "fail" after warm up.
I would point at the radio, this type of fault is not uncommon. Some component is probably failing due to heat. I would guess something in the power-supply, since it's both AM and FM.