Hi dibank,
Welcome to the Heartland Owners Forum and Family. We have a great bunch of people here with lots of information and all willing to share.
The button on the panel labeled "HEATER" is to turn on the propane side of the water heater. When you turn it on always be sure you have water in the heater. The button will turn red and a small little light next to it will be on untill the propane ignites. You should be able to hear the water heater igniter clicking and also the burner when it comes on.
Also when you turn on the water heater electric switch, be sure you have the heater full of water... It will burn out the electric element almost immediately if not full of water.
I am pretty sure your tanks do not have a heater, unless you ordered them and they would be on a separate switch.
Enjoy the forum.
Jim M