Attwood heaters are made of aluminum and don't use an anode. Suburban heaters require an anode because they are glass lined steel and subject to galvanic corrosion.
2007 Tundra DC 4X4 5.7, Alcan custom rear springs, 2009 Cougar 245RKS, 370 watts ET solar, Victron BMV-712, Victron SmartSolar 100/30, 200AH LiP04 bank, ProWatt 2000.