I have an Iconic 2714SF with gas only water heater. I'm pretty sure I could put ice in my hot water tank and in 20 minutes it would be hot and the burner would shut off. When taking a shower using holding tank water the heater will run at best 10 minutes after the water is shut off. You might want to actually time the unit to see if it's really running that long and if it is then something isn't right and you should get it checked out. It's much easier to have the water heater fixed so it's working correctly then add a timer which is not needed.
If you don't want to leave the water heater on while camping then just turn it off at the rocker switch when not needed. I turn my heater and water pump off when leaving the trailer just incase and turn the water heater off at night so I don't have to listen to it burn while trying to sleep. It took a few times but now I'm in the habit of turning off the switches when I walk out the trailer door and turning them on when I first walk in the door after being gone. First thing I do in the morning is turn on the water heater then make coffee for the wife.