the outside switch mounted to the water heater is for the elec. i turn mine on after i have hooked up the water at the camp ground and turn it off when i leave (even when traveling between campgrounds...a habit).
the switch inside has a red light on or next to it.
on an elec/gas water heater, the plug is usually an anode rod. on mine, i have to pull the anode rod to drain the tank.
you may want to look at one of the post on winterizing to see what u may have left to undo.
after i apply water to the trailer, i pull the lever on the relief valve at the top of the hot water heater to make sure it really is full before the electricity goes on. check out the link below.
What are these valves for?