If the bolt you referred to is the drain (for Atwood) or anode (if Suburban) it should have a hex head about size 15/16 (as I remember) and the easiest way to get it out is with a socket. If yours is rusted it may be useful to spray it with WD-40 and, even better, use a hammer drill on the socket.
However, since it is winter now I would suggest removing the relief valve and pour 1/2 bottle of A/F thru the hole. There may not be enough water in the heater to be a problem but this is a little insurance.
Next year, however, remove the drain/anode plug to get all the water and A/F out AND use a Tank Saver sprayer to flush out more of the crud.