I'd verify the size of your current water heater before going much further.
The next consideration is that you'll need a way to turn the electric element on and off. Some water heaters have a switch directly on them for the electric element, while others use a remote controlling switch, typically on the same panel where the switch is for the electronic LP ignition (DSI), if your current water heater has that option. So, you may be running two separate wires. One will be the 12/2 romex cable from the circuit breaker panel to the water heater to power the electric element. This needs to be on it's own 20 amp breaker, and it needs to be 12 gauge, not 14 gauge.
The second wire (or wire pair) will be a 12 volt
wire from the water heater to where you want the controlling switch. Depending on the wiring already in place, you may need to run a return (ground) wire as well to the switch. Read the instructions that come with the water heater to determine the wire gauge needed, but 14 gauge is probably safe as it's a control line, not the power supply line for the element. But again, verify that in the installation instructions.
You may find that your trailer is already wired for a combination electric/LP water heater, which would make it a much easier installation.
Also, if the current water heater has a standing pilot light (not DSI), and you want to upgrade to a DSI unit, then you'll need a control switch and wiring installed for that as well. Keeping the new control switches fairly close to the water heater will make wiring easier.