Hello Harry,
Everything alright on 12V dc, but difficulties with AM reception when on mains, would I be right in assuming it is a car radio that has been fitted?
If so two things come to mind,
Firstly, Car radio's should be fitted with power supply line filters, which are designed to prevent interference from other 12V equipment and the High Tension ignition system in a car from causing
difficulties in the radio. Power line disturbances can upset the AM demodulator resulting in either a lot of background noise, or the audio signal being intermittent. The mains power supply/charger fitted to caravans also can produce power line artefacts that could upset the demodulator. A possible solution would be to fit car type suppressors to the power leads to the radio.
The second point is one I have come across. A caravan standing alone without a mains supply connected, was fine, but as soon as the mains cable (but not turned on) was attached, the radio's signal to noise deteriorated significantly. This was due to he mains cable connecting the shell of the caravan to a better earth and it began to shield the radio from the required signal. The solution was to ensure that a proper external aerial was used.