Our rig is the same. With the un-insulated area in the front and the area behind the pass through practically wide open below where the water heater is, I don't think you will gain much. You have a heat duct that goes into the pass through which helps, and I thought about the same thing to try to close up all the openings, not only for heat but also for mice, but the hassle just was overwhelming, and possibly would cause other problems. You might gain a little if you just used insulating rigid foam board on the ceiling of the pass through, but you will still have the aluminum beams that will conduct heat and cold, but there could be an improvement. Hope you are successful in making it work.
Bob & Becky
2012 Chevy 2500HD D/A CC 4WD