Suburban hot water heater was turned on for the first time on control panel when it wasn't turned to on position at the water heater itself. Did I ruin entire water heater or can I just replace heating element ?
If the switch at the water heater itself was off, no harm should have been done. And, yes; water heater elements can usually be replaced Instead of replacing the whole water heater.
Where problems occur is when the heating element is on with no water in the tank. Be sure your water heater tank is full of water (temporarily open the overflow valve on the heater until water flows out) and then turn both switches on (control panel and water heater). You should have hot water in an hour or two. If you need it more quickly, activate the propane burner too. You can run them both at the same time.
Just remember that you need water in the water heater tank before using the electric element.
You must have the outside switch on and the inside switch on to be on electric. Some travel trailers only turn on the electric from outside. And TJ is spot on to make sure it has water in it before you try to heat it.