Water heater issue?

[Alsatian]

Coming to the end of a few days at Bridlington South Cliff Campsite and I put the {Truma} water heater {electric) on. After 20 minutes it was still cold!
I then noticed that the red 'failure' light was showing on the left hand side. I checked all the fuses and RCDs but couldn't find anything, I tried it again and it worked ok!
The only thing that had changed was that the first time I put it on the habitation aircon was switched on, and the second time I'd switched it off.

Could this be the cause of the failure message?
PS campsite is 16amp hookup.

 

[Lenny HB]

I think red light normally means 12v is too low but I wouldn't expect the aircon to affect the battery voltage when on mains.

Try to replicate the fault while measuring the battery voltage.

 

[dunnah01]

The Truma Combi controls can be a bit lazy occasionally. I need to turn the main control panel off now and then to reset the Combi heater