Both our TT's have had Atwood WH. I believe there is a label stating the manufacturer on the WH someplace and I seem to remember the Atwood "A" being cast into a metal part on the WH. They're aluminum and do not require an anode rod. I've bought replacement plugs at a big box store and at my local RV parts place. They are usually come two to a card.
An anode won't address calcium deposits, they're to prevent galvanic corrosion. I think North is correct about them being 1/2" and they are pipe threaded.
You mention a leak, if it's around the plug I'd try replacing it. Some folks wrap the plug with Teflon tape or slobber some pipe dope on it prior to installing. I don't use anything, but that's just me. I tend to replace the plug when it starts looking "gnawed upon" which is sorta once a season or so.
2013 F-350 CC SB 2WD 6.7PS
2013 Eagle Premier 351 RLTS
-SOLD- 2012 X23B
-SOLD- 2003 Ford Expedition 5.4, Bilstein shocks