I have the same problem. I believe that the glue between the layers of the flooring has failed. Not much you can do without removing floor covering (carpet, vinyl, laminate, ?). If I ever change out the carpet again, I intend to drill holes through the first layer, inject something like liquid nails, add weights until it cures, add some short screws, and hope for the best.
If you could drive short screws through both sections of floor boards it would probably stop the squeaking, but I don't know how to do this without making a mess of the carpet, laminate, etc.
There is a kit for home flooring to use through carpet where the screw breaks off at the top of the flooring and below the carpet and carpet pad, but those screws are much too long for use on a motorhome floor.
If you find a solution, please post the results.
Fred
Fred & Vicki
St. Augustine, Florida