Depends on you model most now have pezo auto ignition pushing the gas knob down to operate the pezo. It may be either a faulty pezo or dirt in the gas burner if its not been used for some time.
An air line blast should clear it.
He said when you have dual fuel ie electric or gas use the gas at least twice during period of use of c/v don't just use mains electric because its convenient and to save gas.
Find out what model the heater is and go to www.truma.com find heating find downloads find your model down load the pdf.
Bob UK
December 2013
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