2013 Berkshire 390 fl. I have an electrical box and a plug in in the battery compartment. What is the plug for

depending on your interior design should be rear driverside utility behind breakers---where fresh water tank located

Touching on what David said, check for the battery disconnect. It should be a red lever located in the front compartments, possibly near the batteries. The plug used for the connection to your truck will only provide power to the parking and brake lights and to the electric brakes on your axles. There will not be any power to your unit. Something else to consider, with the age of your 5th wheel is the life of the batteries. My 2006 KZ needed the first set of batteries replaced in 2013. Use a multimeter to check your voltages or consider taking them to a local Auto parts store to have checked to see if they need recharged or replaced.

At the back of the water heater ( unless you have a bypass outside in the compartment with the sewer and city water )there should be 3 valves, the one on the vertical line leave open and close the 2 leading into the heater. That will bypass your heater. Then connect your antifreeze hose (garden hose ) to the city connect. Using a water pump ( just like the one in your unit) have someone plug it in ( suction hose in the antifreeze of course) open one at a time each faucet hot and then cold till you see the pink running. It may take a couple gallons so get a 5 gal jug. Don't forget to remove the water filter under the sink.

go here and find your diagram, Fleetwood Discovery 34Q Wiring Schematic Bing images

pump is near water tank

The microwave and outlets are on a 115-120vAC source running through the breakers box. The lights are 12vDC running through the converter then to the fuse panel. Locate the inverter in the compartments up front, possibly behind a panel that can slide or screws to hold the panel. Depending on the Mfr if the inverter, you should see a blade style buss fuse on the side, typically near the 12vDC output. Use a multimeter to check for the 120vAC input and 12vDC output. AC power-yes and DC power-no, check the fuse, replace if necessary. If the fuse is good, the inverter has blown and needs to be replaced. If you have AC & DC power on the inverter, go to the batteries and check for voltage without the inverter supplying power. Pull the fuse off the inverter or switch the battery disconnect switch. If the batteries have little to no voltage, cells may be dead. Take the batteries to a local auto parts store to get checked to see if the battery needs a charge or replacement.

Is the system electric or hydraulic? If electric, on the opposite side of the coach from the slide out there will be a place to insert a manual crank handle. It may be in a compartment or under the entry steps. It will be a hole about 1 to 1-1/2" in diameter. If you look inside you will see a solid shaft with a cross pin. You will slide the manual crank handle onto that shaft and begin to crank and the slide will either come in or go out depending on the direction you crank the handle.

connect your disconnect

What kind of thermostat and AC system is in the coach? More than likely there has been a loss of 12v power to the control board in the AC which is where the thermostat receives its power.