Hot Water Tank Smell

my HW tank is the kind that does not have an anode rod. I've had this trailer for 13 years this August and seems like the last couple years it has done this. It probably just needs a good bleaching. I drain it when I winterize it. The drain plug is hard to get to and we installed a relief valve type plug last fall and it takes a day or so to drain. The old plastic plug wouldn't come out. I don't think I added any bleach to the fresh water tank when I used it the first time this spring, so may be why ? It's only been 2 months and it's stored in a cool barn when not in use.