Truma CP Plus panel problem

Habitation Isolator switch has been off since last trip away. Fitted a new battery. Solar display shows normal output. Isolator switch to ON. All 12v electrics appear OK. Truma panel normally lights up when Isolator switched ON. Completely blank. Panel off, remove data cable, check power supply = 0. Turn Isolator OFF, power supply then reads 12.9v. Panel still blank. Connect van to mains and repeat earlier checks - same result. Reluctant to shell out £150 for new panel if that isn't the problem. Complex electrics are not really my forte so any guidance would be welcome. If I am able to resolve this, I don't want a repeat when I link up the second battery in a week or so when the voltages and charge levels are equalised, although I can't see why swapping a battery in the van should have affected it.