Tankless water heaters are designed for a specific water temperature difference and flow rate. If you match that, they are fine. My experience has been variable. I've used a tankless heater in the tropics that delivered steam to the kitchen sink. I've used them in hotel bathrooms in Europe and Asia that did a fine job keeping up with showers. I have no experience with RV units but have heard complaints about fluctuating temperatures.
Even tanked heaters can be a problem. I run my gas water heater at home at a temperature where I can be comfortable with hot only for a shower, but if I run hot water long enough that the burner comes on, the water delivered gets too hot, because the thermostat is at the bottom of the tank.
For tankless, BTU inputs are usually constant, only choices are "off" and "on" and if "on" is adequate it can be too much; if not adequate, water is too cold. A heater with a tank has a reservoir of heated water at a lnown temperature, you can mix it with cold for a steady lower temperature, but as you deplete the tank,you are at the mercy of the heating system,as with tankless.
I'm not aware of any tankless heater systems targeted to the RV market that have heating rates much different than recovery rates for 6-10 gallon RV water heaters, all of which heat water to a higher temperature than I prefer, but if the volume is small, maybe that's the way it has to be.