Water Heater pilot light will not stay lit?

I have a 92 Layton 5th wheel and the pilot light on the water heater will not stay lit. I changed the thermo couple, but the only one that I could find had a 18 inch wire on it. The original one had a 10 inch. I can light the pilot, but it will not,stay lit unless I hold my lighter to the thermo couple. The burner will light and heat up the water, but after it shuts off and cools down the I can hear the magnetic switch click off and the pilot will go out. I was able to bend the thermocouple holder a little, but probably 75% of the time the pilot will go out. If I can find a thermo couple with a shorter wire on it would it work better? Or is there something else that I need to try?

Bob