You really should remove that plastic plug and give the tank a good flushing. Sediment and mineral buildup over time and add to the rotten smell.
Use one of these to thoroughly flush the tank. Available at most RV accessory parts stores.
Tank FlusherMost stuck or rounded off plastic drain plugs occur from over tightening. I put a couple of wraps of Teflon pipe tape around the threads of the plastic plug and do not keep wrenching on it until it stops turning. A little tighter than just snug is enough. Although I carry a spare, I have been using the same plastic drain plug for 10 years.