Just went through a similar problem with my new TT and the water heater. There are no controls on the water heater, simply an on/off switch in the trailer. Turn it on, it will try to light three times then stop. The light by the switch will come on telling you it did not light properly. Mine would light the first time on a tank of cold water but would not restart to keep the water hot after use. No gas issues, just would not stay lit after the first start. Short version the dealer had to replace the control board on the water heater.
Net - don't assume that if it lights the first time it will again. Cycle it to test. Let it get to temp, shut off, then use some hot water and make sure it will re-light a few times. I made a couple of trips thinking it was fixed. Would get to camp site and light the water heater. I would take the first shower and then no hot water for my wife to use or for washing dishes, etc.