http://www.allencampermfg.com/Up-to-29--Travel-Trailers.html Check out these small TTs.
I doubt you will find "fully heated" tanks on a small RV trailer.
I had a 27' trailer that had "heated" tanks. All they did was make a 4" hole in the floor for a 2" drain pipe above each tank for heated air from inside the trailer to flow down and around the tanks. The tanks had a plastic cover on the bottom and sides but was open at the top. Huge source of cold air into the trailer in the winter.
The trailer had two grey and a black tank. I closed of the holes in the floor and had my RV dealer encapsulate each tank with foam and a electric tank blanket under the bottom. We lived in this RV for seven months one winter with temps down to single digit and 40 mph winds. Nothing frozen.