As said has the battery been checked.  It is at the bottom underneath.  I think you may need to remove the from cowling to access it? This just clips off.   Can someone confirm?

All the Truma heaters I had were AA battery powered,I found out the hard way of freezing one New Year...lift front off and should be a little holder at floor level....replace battery and try again. If it isn't this it could be the thermocouple...but would think anyone with an savvy doing servicing would know this?? geoff