It would have been helpful to know what brand of water heater you have, Surburban, Atwood, etc.
If you have a Surburban, the drain plug is normally the anode rod that is removed to drain the water.
If you have an Atwood which normally is manufactured with an aluminum tank and should have a nylon drain plug and NEVER use a steel or galvanized plug to replace the nylon plug. This is because with the dissimilar metals, (aluminum to steel) you will see an electrolysis chemical reaction that, over time, will virtually seize these two components together, and make it extremely hard to break the plug loose from the tank.
One trick might be, if there is room, to apply a little heat to the tank, around the plug, and put ice directly on the plug and then use a shock load or impact to try to break the plug loose from the tank.
When you get the plug out, I would recommend using a pipe tap to chase the threads in the tank and use a new plug, with Teflon tape to secure it, and only tighten it enough to assure it won't leak, so it is easier to get out the next time you want to drain the tank.
Hope this helps and happy RV'g