I don't have a tankless in my trailer but I do have one - Stiebel - 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 traveled 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.
Here we go again.
Suddenly I decided that tankless might be a nice thing to try, so I did my research and came up with a strange - still interesting - solution.
The small water heater at home depot was my first choice, but with the pressure relief valve on top it is too tall to fit in a cabinet in my camper. They have to be mounted upright in order for the relief valve to protect you from an explosion. I was discussing the same issue on another forum, it seems to be a good solution, but Ariston is not an option since it's a bit more than I want to spend.
What I settled on --and it is going in this weekend-- is an Instant-Hot water dispenser by Insinkerator (3d). It is normally used at a kitchen sink to provide very hot water for tea or coffee. I plan to install it just for the purpose of washing hands and dishes, thinking I will run some hot water in the sink and then mix in cold to make it tolerable (as it heats to almost boiling temperatures).
Once again, I'll be the first to admit that this - (and other tankless options) - is not the ideal solution for getting hot water in your sink, and it certainly will not provide enough hot water for a shower, even with a mixing valve (though I'll test it as well). But, it is small and will fit where I need it to fit.
The last - but not the least! - the thing only draws 750 watts.