How old is the control?
Using the enter button check the voltage of the 2 batteries, you can scroll through 'bat' and 'car', bat shows the leisure battery voltage, 'car' shows the cars voltage. When the unit goes into charge mode, the voltage on the leisure battery should be the roughly the same as the voltage coming from the car.
If the two are not equal and the unit is saying CHG, then there is likely something wrong with the relay.
Ian