Holding Tank Heating?

I have a hand-me-down 2004 Sunnybrook Titan 28RLKS 5th wheel trailer. The literature on this, and other Sunnybrook trailers of this era, talks about insulated and heated holding tanks. But if the heated part is true, I can't find any way to turn it on or off. Does anyone here know anything about whether there are tank heaters or not and, if so, how they are operated? Or is this just about warm air circulation when the interior heater is on? It is a nice trailer (with a few age related issues) that seems to pull fine. I am just trying to figure it out.

JRMunn