The lights on a trailer are almost certainly all 12 volts so you need to look for a blown fuse not a popped circuit breaker. If the fuses are all good you'll have to start looking for a broken connection. Either a wire is disconnected or more likely a failed ground connection. Since it's more than one light it might be in the wire loom near the fuse block.
The hot water tank should have a manual valve on the rear. That controls the outflow. Look for one with either a blade style or crank style handle and open it. There may even be two manual valves back there, open both in case the trailer manufacturer got a bit "anal" and put in a second one that is really not needed.
To fill the water heater you open the "pop off" pressure valve on the top of the outside of the heater and allow the water from the trailer to fill the unit. That has a small lever on it and controls a spring loaded valve. It is designed to release pressure and prevent the tank from bursting if it gets too hot. Once water starts to trickle out of it, close the valve. That leaves a small air pocket in the tank that you need for expansion as the water heats. From that point as long as the manual valve is in the open position it will self regulate. If the water heater pressure valve dumps water when the heater is hot you have over filled it. Simply shut off the water at the supply point for the trailer, then open the pressure valve and let some of the water flow out. When the flow reduces to a trickle close it and you should be good to go. Turn the water for the rig back on and you will be OK.
I hope you have not turned on the heat in the water heater before filling it. If you have you may have blown out the electrical heating element or overheated the gas burner tube. Both are a bad thing. Never operate the water heater without filling it first.
Mike Nancy and the fuzzies
Fulltiming since June 2004
Volvo 660 MH tow vehicle
2005 MS 38RL
2007 Saturn Ion "toad"
2010 Gold Wing "piggyback"