Keystone used to have historical brochures available on line, they may give construction practices. If you pull a register and peal back the layers you should expose the floor materials. The floor will have a Joist on each side of the register and the heat duct running between the joist front to back. With all that said there is no support for the flooring material at the register due to the duct lying just below it. In other area of the flooring there should be foam inserts.
https://www.keystonerv.com/media/2405/laredo-2012.pdf
There are YouTube video's showing Keystone floor construction.