Suburban water heaters have a rocker switch outside. You have to remove the outside cover to get to it. It's a safety switch so that if you winterize and drain the water heater, you can't accidentally burn out the heating element by turning on the inside switch with no water in the water heater tank.
So make sure you have water inside the tank. Trailers are winterized when they leave the factory, so there's no guarantee of water in the tank unless you know for sure that the dealer took care of that, or you did. With city water hooked up, you can open the pressure relief valve on the water heater to check. Just be careful you don't get scalded by hot water in case it really was on all the time.