We were also plagued by the ridiculously short dump hose carrier phenomenon - why do manufacturers do this??
Instead of building one from scratch, we modified the existing Valterra contraption by lengthening it. Blog post with instructions here
, and here's a pic showing the extension. That portion partly to the right of the metal plate is what we added - you can see the seam near the left edge of this photo. That's where the old tube stopped. It's PVC but we painted it black.