Just cause 'THEY' advised you should use one.....THEY are WRONG!
Lots of items ar available via aftermarket but that has no relationship to actual need.
Atwood doesn't supply one OEM..doesn't have an Atwood part number and states they are NOT required.
Do the vinegar soak.
Remove T&P Relief Valve
Install NEW NYLON drain plug...Atwood uses NYLON due to high temp rating (Plastic temp rating is NOT high enough)
10 gal WH tank------6 gallons of vinegar/4 gallons water
Use funnel and fill WH tank with mixture
Install T&P Relief Valve
Run WH thru 4-5 heat cycles W/O using/adding any water
Heat/let cool....repeat
Then drain WH and flush
(Do a 'power flush' but removing drain plug and then turning water on full and let it BLAST out of drain hole.....drain hole is only 1/2")