What do you mean by "on"? Manual valves or electric switches or both?
My Sprinter 5er has a propane-only water heater. There are two parts to turning it on.
1] The water must be turned on. There is a winterization (diverter) valve near the water heater tank that must be turned on so water will flow though the heater.
In winter mode, you want to add antifreeze to the water lines, but not to the water that flows through the water heater. So you turn off the water to the water heater by that winiterization valve and then drain the heater for wintertime storage. Then pour RV antifreeze into the fresh water tank and pump it though the system until the pink stuff flows out of all faucets and toilet.
Then before the first trip in the spring, flush out the fresh water tank and then turn on the water to the water heater and flush out the lines to get rid of the pink stuff.
2] The propane must be turned on. The valve is on the top of each propane tank. If one of them is not turned on, then you won't have any hot water fired by propane. The propane-only water heater should be "automatic" in that it will fire up anytime you have cold water and the propane is turned on - provided you have purged the air out of the gas line after changing propane tanks. If the propane is not already turned on, then you may need to purge the gas line by lighting the gas range and turn on all the burners until it gets all the air out of the line. Then the automatic firing mechanism should work on the heater after a few minutes of click-click-click waiting until there is gas instead of air at the burner.
Newer trailers might have a 12 volt/propane heating system. If so, you must find the switches and turn on one of both of the water heater switches. In our new TT, the control panel is up high on a cabinet over the kitchen sink. You can turn on or off either one or both of the electric or propane heating elements in the water heater.
We find that expensive propane is nice to have only if you don't have any hot water and you need a bath real soon now. In that case, turn on both the propane and 12-volt heaters at the same time. Otherwise (if you are plugged into 120-volt shore line) the electric part of the water heater system is just fine without using any propane.
The electric heater in the water heater will suck the juice out of the RV battery in a heartbeat, so you don't want to use it unless you are plugged into shore power, such as at a campground. For boondog camping, use propane only.
Grumpy ole man with over 50 years towing experience. Now my heaviest trailer is a 7,000-pound enclosed cargo trailer, RV is a 5,600 pound Skyline Nomad Joey 196S, and my tow vehicle is a 2012 F-150 EcoBoost SuperCrew.