Here's a thread I asked on the same subject a while back: https://www.forestriverforums.com/fo...do-195942.html
As far as I can tell, on the GT5 it's the tank heating pads and the drain elbow heater pads plus those black plastic panels you see when you open the compartment door that holds the water pump and the compartment door next to the water heater (where the water heater bypass valves are located). There also is some kind of foil insulation in a few compartments.
From experimentation and monitoring I can tell you that on our GT5 the compartment that holds the water pump stays about ten degrees above the outside air temperature. So if it's 35 degrees outside then the water pump compartment temperature is about 45 degrees.
The wet bay compartment with the water filter is a different story. Even though ours is underneath the shower and there is a propane furnace duct that runs underneath the shower, there is no furnace heated air piped into the wet bay. The wet bay is maybe three degrees above ambient. Maybe.
I bought a small 250-watt electric heater and I place it in the wet bay when the temps are getting near freezing. Because the heater has a safety switch on the bottom I place the heater on a small board so it sits on a flat area.
I use a Thermo Cube to control the heater. The Thermo Cube plugs into an extension cord and has a thermal switch in it. When the temp in the wet bay drops to 35 degrees the Thermo Cube turns that 250-watt heater on. When the temp gets to 45 degrees the Thermo Cube turns the heater off.
For power, I plug the extension into an outlet we have in a basement compartment on the passenger side. I then run the extension cord through the pass-through to the driver's side. Then it goes out that compartment through access plate in the floor and up into the wet bay.
One thing to note. You may want to run the propane furnace rather than electric heaters when the temps get close to freezing. On ours, the heating ducts run near the water lines on the driver's side so heat loss from those ducts will help keep the water lines warmer. That does not help us on the passenger side or in the kitchen so we keep those drawers and cabinet doors open a bit to allow warmer air to get into those areas.