I just did this with a tankless water heater demanding 40A. Your system requires 120VAC @ 1400W. The Ohm's Law equation, I = P/E confirms that your water heater will need 11.67A, RMS of power for operation: I = 1400W/120VAC.
If I was you, I'd go purchase a single pole <manufacturer specific> 15A breaker to install in the box; this will protect your water heater. Now you will need to add the circuit. Run a new wire (12-2 gauge will be plenty, "mc-lite" cable preferable for outdoor locations) from the water heater to the distribution box, and then connect: white to the white common, copper or green to the ground common; then connect the black cable to the output of the breaker, and plug it in. I see someone has already tied in the second breaker position with that red wire in your photo of your box so that should be all you need to have your power on the other end where your water heater is. Follow their instruction for connection, and you'll be up and running in no time.
Adding an additional load of <12A at the distance you are stretching it should not be an issue. At worst you'll blow the resettable GFI outlet or park breaker if they aren't supplying enough current to meet your demands. You won't trip anything on your end. To manage that feat, you'll need to have everything on: your AC, your water heater, microwave, chargers, a vaccuum, and then probably a other things...
If you are truly worried about it... all you would have to do is run a new beefier cable to your distribution box; a 6-2 nmb should do ya.' Kirchoff's current law reveals the additional current load would only be exuded by your trailer plug in cable and no where else; aka just because you add a 12A device doesn't mean it will affect any other wiring in the trailer. This, of course, won't solve the problem if you are demanding more than 30A of service, but even with your water heater system installed, you'll have to try pretty hard I think...
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