Truma water heater problem.

Old firefighter

We've bought a 2004 Lunar Stellar & wonder if anyone can advise why the water heater wont operate with gas ?   It gives piping hot water with the electric supply, but fails when I try the gas operation. I follow the instructions & turn the control
to a heat setting, & rotate to the flame position.  A green light illuminates, followed after 20 seconds or so by a red 'failure' light.  I've tried resetting after various time lapses, double checked the gas sequence as advised, without success.  I bought
the van from the original owner, who states he always used mains hookup, & had never tried the gas during his 11yrs ownership !  Any suggestions please ?