I successfully altered fitted sheets to fit my rv mattress. Queen Rv mattress was 6” shorter than what we use in the house. (I determined that with a tape measure. ). I sewed a 3” tuck from side to side right through the middle. Visualize this: after sewing I have a 3” flap (by that I have shortened the sheet 6”.). Then I removed 2” of the flap, leaving 1”. I cut just ONE of the sides again to 1/2” and folded the 1” over the 1/2” and pinned it in place then stitched through all thicknesses. This makes a Felt Fell seam. Raw edge is encased just like the side seam down your leg of jeans. Sheet fits perfect & you never notice the alteration. I had extra sheets on hand so no expense to me.