Sounds like you’ve got a dead short somewhere, or a bad breaker. I’d try disconnecting the hot water heater, tape or wire-nut the ends of the 110 vac feed wire, and flip on the breaker again. If the breaker still trips right away, then you need to work back towards the breaker. Do the same thing at the 110 vac switch that controls the water heater. If the breaker still trips, continue with the next step. If the breaker doesn’t trip, then either the switch is bad, or the wiring between the switch and the water heater is shorted.
Next, I’d unplug the trailer, and remove the breaker. If you have an ohm meter, check for a short on the black wire that was connected to the breaker and either the white neutral wire or ground. With the water heater still disconnected, that should read as an open circuit. If it reads zero or close to zero ohms, the short is in the wire itself. Perhaps it shorted where it passes through the frame or skin somewhere. Won’t be fun to track down in that case. Might be easier to just disconnect both ends of the wire, and run a new one from the breaker box to the water heater.
Of course, you could just replace the breaker as step one and see if that’s the problem. That is really the easiest thing to do, and if it works, you’ll be done in just a few minutes. If it doesn’t work, you’re out the cost of the replacement breaker.
Good luck, and be careful!