I have not inspected the newest mobile homes, but some older ones can benefit from a simple duct sealing project. As the unit is built, the underfloor ducts are placed, the decking panels are installed and then the register openings are cut in both the decking and the ductwork. The register boots have the flanges spread at the bottom, but not really sealed to the ductwork. A mirror and light can inspect this installation. To seal this area, first press the flanges tight against the duct with fingers. Then use duct seal to close the area between the duct and the boot flanges. Duct seal is about the consistency of mashed potatoes, purchased in small buckets or packaged in caulking tubes. Apply the duct seal with fingers- messy operation. It may take two or three sessions to fill all the gaps, with a few days for the previous application to dry/harden. Inspect with mirror as necessary. I predict an improvement in efficiency of the system of 5 to 10%.