I don't have a tankless in my trailer but I do have one in my house. We have a recirculating pump that is also installed. What this does is take the water that is already in the lines and recirculates it back through the heater. It will do this until the water in the lines is hot thus minimizing the amount of water that is wasted.
As for a temperature, it works pretty well. I travelled with Truma heater previously, so twice that henryck said. Usually it takes some time to heat the water, but there could be an option to keep the temperature at the certain level.