I was under the impression it was for all three tanks. They are just for the tank only, not the bay.
Depending on where you reside and the weather, the furnace (not heat pump) will do the job just fine. We have spent several days in our coach with night time temps into the low 20's and daytime temps above freezing with no issues and have not used the heaters.
Others have also installed a light in the compartment to keep things thawed out.
2013 Berkshire 390BH / 360hp