Here is the sizing chart for an Eemax: http://www.grainger.com/ec/pdf/Tankl...zing-Guide.pdf
Their stated application (Eemax SS023240TC) would need a 240V power supply. It "would be best suited for the single sink and a standard shower with an incoming average water temperature of 58°F
. So the climate where you are is a determinant. I've used tankless water heaters in my houses for 20 years, and love them, but in the winter, when the incoming water is colder, I have to reduce the flow greatly to have hot water. When my present unit stops working, I'll replace it with a larger 240V unit so I don't have to do this, but that's not an option for a trailer.
As I stated, if you want hot (not warm) water, either an rv heater with a tank, or an rv tankless heater would be the way to go. Both pieces of equipment are more expensive than your on-demand unit, but you will have to install a dedicated circuit for it, which will cost some money, and you probably won't be able to use it off-grid.
If you do want to go with an on-demand, PrecisionTemp, Atwood and Girard make them for rv's. PrecisionTemp is very pricey, and I believe the others have prices about twice those of replacement tanked heaters.