With DP windows, you have a nice start.
I don't have them, but I made some improvements to my TT.
We had some temps in the single digits on one trip, and though the heater kept it warm inside, we had some frozen pipes. Evidently the factory heated and enclosed tanks needed some help.
I installed heat tape and insulation on ALL of the FW plumbing. Installed a 120 circuit with a inside switch to turn them on/off. Each heat tape has it's own T-stat that comes on around 38 and off around 45 IIRCC. I then installed 1" polycyronate (sp) which has a R7 insulation rating. It is on the inside of the frame rails, and across the bottom of the underbelly. Basically I made a large insulated box under the floor of the TT. I also added another heat duct to this area.
Though we haven't seen temps that low again, I am confident that when we do, we will have running water. I have noticed that the insulating has made a signifignant reduction in our propane consumption, and the AC works better in the summer too. As an aside, we do run a dehumidifier with the heater.
Total cost of this project was about 400.00. I bought the heat tape on ebay at a good price.