How to turn Water heater on ?

Get the model number off the Atwood heater you have and download the manual. You should have an electric ignitor on that year of trailer controlled by an illuminated rocker switch normally.

Inside rv ,,,locate backside of water heater.

Check valving on water heater tank
Cold inlet OPEN........allowing water to go into WH tank
Hot outlet OPEN........allowing water to come out of WH Tank (may not have a valve,,,,,,may only have a built-in check valve)
Bypass..this valve connect cold inlet with hot outlet CLOSE it

Fill water heater tank (either by turning on hose connected to side of trailer or by turning on trailer water pump (fresh water tank has to be filled)
Then open hot/cold faucets to purge air from plumbing lines...hot will take longer cause you are filling up WH tank

When WH tank is full you can then light off water heater

Depends on what brand water heater and what type Pilot, pilot relite, electronic propane & electric etc.

Instructions should be on panel in outside water heater compartment.

Some electric ON/OFF are in outside compartment (may have an addition ON/OFF switch inside.....MAY) Suburban Brand
Some electric ON/OFF are on backside of water heater tank (where valving is)
Atwoods 2003 year and previous models

Some have 'gas' ON/OFF on panel with light inside RV
Some have pilot flames that have to be lit from outside compartment

I hope it is winterized otherwise you would have a real problem in Minnesota winters.

My heater on/off switch is beside my generator switch. If you have an electric heater, do NOT turn it on until you have filled it with water. Even if it is propane only, do not turn it on until it is full.

If it's winterized, the water heater should have been emptied and by-passed....there is no bypass "switch", there are manual turn valves in-line with the water lines that shut the supply off to the water heater. The bypass valves will be behind the water heater either in a storage compartment or in a cabinet cavity. You will need to put the water heater drain plug back in then open the bypass valves to get water in the heater. After the heater is filled you will purge the air out of the tank using the relief valve. Once those steps are complete, then you can try the heater on gar or electric.

Hello, it is a 2003 Forest River Salem 26ft toy hauler. ( I will have to add that to my Sig )

I bought the camper from someone at my work, who no longer works here. When I bought it, he had it "winterized" by the dealership,

I have no "bypass" switch anywhere in the camper that I could see. not by the water pump switch, or anywhere else obvious.

I have opened the outside door that shows the water heater, I will have to look this weekend when I go up there . ThankS !

Mine has a toggle switch in the compartment with the water heater. It was hard to spot and gave me fits for awhile trying to find it.

Some (mostly older) RV water heaters need to have a pilot light lit, kind of like (older) residential water heaters. Most newer ones have some sort of electronic ignition and have a switch in the camper somewhere to turn on the control board and thermostat. On mine, it's next to the switch for the water pump on the little tank level indicator and control panel, and labeled "heater."

Make sure you have it filled with water (i.e. not bypassed) before trying to fire it up. As others have said, it may need cleaning or repair after three years. You may also have a weepy pressure relief valve, which may or may not start sealing better with a bit of time/use. A little bit of dripping from the relief valve is generally nothing to be too concerned about.

It would also be a good idea to check for things like wasp nests before starting the heater.

Since you've haven't used the water heater in 3 years, if it's propane, it most likely won't work anyway. Things need to be used or they rust and stop working.

First, if you've never used it in 3 years, and you're in MN, then it's probably empty. you'll need to find the bypass and turn it so water can get to it.

Sitting for 3 years, you may need some replacement parts. Open the outside panel and have a look to make sure the propane delivery and the lighting element look good.

There should be a switch somewhere that you'll need to flip to turn it on, probably in the kitchen area. Mine is right beside the water pump switch, not sure if all are built similarly.

If you find the switch, I'd head outside right after I flip it and have a look over the heating area for the tank, just to make sure all looks fine as it lights (if it lights).

Since we don't know what type of Rv you have I'll take a guess. You have either an LP or electric water heater or both. Provided it's not bypassed (usually done to winterize), it should be as simple as a flipping a switch. Usually this is on your control panel - same place as your tank readings etc. If it's gas and electric you may have a switch for each (electric if you are plugged in , gas if you are not on shore power). flip the switch, wait 15 mins and you should have hot water.