Assume other gas appliances are working...Stove & Fridge on gas. If yes, check gas tap open to Water Heater, otherwise check if jet is blocked.

My thread re same/similar problem.

http://thegreynomads.activeboard.com/t62526988/hot-water-problem-on-gas/

Aussie Paul. 

Hi Perry

Rub you finger across the jet out let , it does not work, push a very fine wire into the hole to clear any debris.  Still no go, get it checked out. 

Cheers

Hi Perry,

Just had the same thing on both our van, as well as that of a friend. There's two things you need to do...

1. Using an air hose, blow out the combustion chamber and the fire tube. You can do this in situ without dismantling anything. You will probably blow out a lot of rust and scale.

2. This bit though is the more important step. Unplug the fat red wire from the spark igniter/flame sensor and remove the sensor. There's one screw to undo and it pulls straight out. Using a fine emery or even a nail file, clean the tips to remove any scale, back to a bright-ish metal surface. DO NOT bend or adjust the rods or the gap between them. Replace the unit and reconnect the wire (spade terminal) making sure it is clean and tight.

That should fix it for you for another 4 or 5 years. Good luck.