RV Water heater - servicing and repair
First, the problem may be simply a lack of gas at the burner. It may take a few tries, especially after it's been in storage for a time, to get the gas up to the burner. I usually recommend that you light a stove burner first (after bringing the unit out of storage) to get the gas flowing in the lines.
A lot of water heater problems are caused by poor electrical connections. The water heater is "out in the elements", so to speak, and the connections need to be clean and solid. Clean all contacts with electrical contact cleaner, make sure the are tight fitting and coat them with dielectric grease to help prevent future problems.
Blow out the burner and flue with compressed air (don't forget the safety glasses!)
Next remove the hex head screw that holds the burner tube to the flue projection. Clean the area where the screw contacts the tube and the flue to make sure you are getting a good ground connection. Clean the contacts on the circuit board and the connectors with electrical contact cleaner.
Then run a soft brush or a clean rag through the burner tube. Often there will be a spider web in there that even compressed air does not dislodge. Take care not to bend the flame spreader (if yours has one). Its a small metal plate located at the end of the burner tube.
Reassemble everything and check to see that you have a good strong blue flame and that the sensor electrode is in the flame (it should glow red after a few seconds.) Check the electrode gap - it should be about 1/8". If you need to adjust it, bend only the ground electrode - not the one in the ceramic sleeve.
See also: RV Hot Water Systems.