We don't have a Silver Shadow and I don't know much about them.
But we do have under floor storage and we love it!

Don't know what we would do without it! I designed it to hold two plastic storage totes designed for bed linens in your bedroom (under bed storage totes 34 1/2"x 18 1/2" x 7 1/2" deep). Under floor Storage area is 36"x44"x8" (his & hers)
Our Mattress is three piece, with a cloth hinge cover. The lids are constructed from light weight aluminium sq. tubing and 1/8" aluminium covers hinged in the middle with a piano hinge for easy access. Small finger hole is all that is required to lift them. My only regret is that they don't have gas rams to hold them up, but I was not willing to sacrifice the space loss.
Under floor storage is at the top of the list for our next build.
Good Roads
Brian & Sandi