As you've probably found, most bike covers for motorhomes are designed for use with a rack on the rear wall of the van, which means a large gap at the back.
Having read reviews about how flimsy some of the alternatives were, I eventually bought
this one
It has survived frequent use, including two trips to France and Spain, no rips or other damage.
My mate used to have one he got from Taylormade (the company that make the windscreen covers) - it was very expensive, and very sturdy. In fact it was too sturdy, it weighed a lot and the thick material made it very difficult to handle and store. It was also far too large, but he may have ordered the wrong size!