If you have an Atwood an anode is not required as it's a coated tank. If you have a Suburban it's required & they go by the thread size, usually 3/4" & there are a couple different materials, aluminum & magnesium & I couldn't tell that one was better than the other. Once you remove it you'll notice it's covered in white scaly deposits & was originally about the diameter of your finger & about 8" long, if it's not replace it, if it's just scaled up you can wire brush it & reuse for a while.
The 2015 Rushmore's have a 12-gallon Suburban water heater that does take the anode rod. Due to the 12-gallons, mine took the longer of two anode rod lengths that most rv stores carry. Hope this helps...
2015 Rushmore Springfield
Disc Brake upgrade, Goodyear G614 tire upgrade
RV FLEX-ARMOR roof, Mor-Ryde IS suspension