I have a 2012 Bighorn and snowbird in Florida at a resort that has well water, the first year I used just two whole house filters and could see the calcium. The last two years I have had a water softener and the two filters processing the water before it gets to the Bighorn - I chew up about 60 - 70% of a rod in a 4 month season. We do have a w/D so we are doing laundry, but it is a front loader so water use is low. It's all in the water for sure. I don't know why a new trailer would have a expired rod in the water heater, that is very curious. Don't know how redrider77 got that amount of time out of his. A reminder to everyone is flush that tank out every time you replace the rod, all the stuff that came off the rod is in the tank it doesn't dissolve.
My first TT had an Atwood and you didn't have to fret over rods - which my Bighorn did.