Appreciate all the responses.
Just bought our new TH a little over 2 months ago. Obviously not much need for tank heaters this time of the year, especially in AZ. So, the tank heaters were the last system I got around to checking.
Called Dutchmen trying to get some clarification on how the tank heaters work---they were clueless. Said to "flip the switch to on" and it should "work". Called our local dealer where we purchased it. Since they sell/service units from 20-30 different manufacturers, they couldn't tell me definitively whether there was a thermostat or not. They said it bring it in and they'll check it out.
Think I may let this slide until winter. Warranty doesn't end till spring. If it doesn't show some current this winter when temps get lower, guess I'll take it in. We're not at all fond of cold weather, so it's unlikely we'll ever need or use the tank heaters . . . but, ya never know.