Switching from elec to gas on water heater

[stotaman]

We have a 30' Sunnybrook travel trailer which we ended up purchasing used late last year (2010). The water heater will work on either gas or elec. When I opened up this spring and tried to use the gas, the unit would fire, ignite and then turn off (while the igniter kept clicking). After trying all combination of electrical swithes, breakers, etc, I finally opted to run on electric. It worked good all season but as I was finishing up the season I asked the previous owner (me next door neighbor) if he ever used gas for the water heater. He replied yes. I then turned the elec switch to the OFF position and powered up the gas switch - along with using some hot water to trip the relay. The gas fired up and held for one cycle and now will not light and will not even click. I don't have the model info with me but it is a 6gal Atwood. I went back to electric and the heater works ok. I could find no directions on which switches and/or breakers to set when switching between gas and electric. In my past camper I replaced both the ignitor and the circuit board, along with the thermistors.
What am I doing wrong? And how do I fix it so I can use gas reliably?

["007"]

You may need to clean the tube from gas valve to igniter, they do get spider webs in them.
Check also the ceramic flame sensor it may have a carbon buildup clean with soft cloth.

[POPPASMURF]

As well you might want to try to clean the flame sensor with fine green scrub or steel wool. The more scratches you put in it the shorter the time to next carboning and failure. Better cure is to replace $20.00 sensor- avaiable from most good HVAC control wholesalers. Turn the power off to service the unit, and then back on. While in there clean out the spider webs as previously mentioned.

[stotaman]

Thanks for all the suggestions. We have closed up for the season but I will do some cleaning in the spring and take it from there. I don't have internet at camp so we stop home every few days to pick up mail and messages. Wish I had found this site a long time ago.