Depends on WHEN you want hot water......
I want hot water WHEN I want it not 15-20 minutes later
Atwood uses a 10,000btu burner and will heat water to 140*F (T-stat OPEN) from 70*F at a rate of 10 gallons per hour
Once heated to 140*F the water temp has to drop below 110*F (T-stat CLOSE) for reheating cycle.
(Gallon of propane has 91,600 BTUs)
Atwood WH Tank has a 'insulation cover' which does a fair job of holding temp
I turn my ON when I set up and leave it ON until I 'break camp' be it 1 day or 2 weeks etc
Again...WHEN do YOU want Hot water?