All Surburban RV Hot Water Systems have an anode rod which absorbs all the corrosive action of hot water. It is located on the bottom of the HWS under the main burner, you simply unscrew this to drain the tank of water. If the rod is mostly eaten away, simply replace it with a new one.
The Suburban Anode Rod is made for use in the 4, 5 & 6 Gallon tanks and feature a 304 stainless steel core for increased durability. Please be aware that some imitation anode rods can cause damage to your Suburban HWS.

Specifications
Dimensions:
Overall: 238mm
Thread Size: 3/4″ NPT