Well I let my sacrificial anode rod in my water heater go a lot longer than I should have, which can lead to early failure of the water heater or internal components and the tank rusting out. So I let it go about 18 months or so and it should have been done at about 1 year. The photo here show a new one on the left and my nearly non-existent one on the right.
I can only attribute the extended life to the fact that I have a complex water filter system in place that eliminates a lot of chlorine, calcium, and other crud but none the less it doesn’t eliminate all of the stuff that can cause problems in the water heater. Simply put it’s best not to put off a simple and cheep annual maintenance item that could lead to costly repairs.
So if you have a Suburban brand water heater in your RV, change the sacrificial anode once a year! If you have a Atwood, they don’t use one because the tank is made of aluminum, and with other types of systems like an Oasis, or Aqua-Hot they need annual maintenance as well. Maintenance that is recommended, is done so to keep things working properly longer, which in the long run means lower costs to you and more enjoyment from your RV.