I pre-wash all fabrics in cold water, very occasionally maybe tepid water (to just get the chill off if it is winter) and will fit in a sink. I don't tumble dry fabrics either - I have never been told whether or not to use hot or cold water nor have I seen it written on fabric bolts. Cold water is recommended in many books. All of the comments received here are very interesting and useful, so my eyes will be more open next time I buy backing fabrics; however, I used good fabrics for quilts 25 years ago before wide backing fabrics were known, but now use sheets mostly, because of ready sizes and cost.
That was certainly a lot of shrinkage and hope your quilt turns out just right in the end.