I've installed electric on demand water heaters in residential , commercial and industrial applications . In order to get HOT water at a sufficient flow rate and obtain the necessary temperature rise for a shower you would need a large unit.
The ones we installed for showers drew 30 to 60 amps at 240 VAC. Since your trailer electrical
service is only 30 amps at 120 VAC , there is a problem .
We often installed tempering tanks to prewarm the water going into the on demand water heater. Ground water temperatures are often in the 40 to 50 deg F range
Check the flow rate and temperature rise of the heater you want to use. May work well for sink use but not for showering.
on demand water heater may be a better solution because the BTU imput is much higher than an electric unit.