For us (and I think for most), the first step was finding the right floorplan for our needs. There were a couple of brands with similar floorplans but, to us, the Grand Designs just felt more solid with better construction than other's we looked at. Once we were leaning toward GDRV, we researched their warranty, customer service and support. We wanted to make sure that the warranty did not preclude full time living and also that they would work directly with us on any warranty issues, with a mobile tech or sending us parts directly. We also asked around on the Grand Design Owners Facebook page. All of that checked out and we went with it. We've not regretted our choice one bit! Our home has been wonderful and the minor issues we've had were handled directly as they promised.
If you're debating on which brand, what we recommend is to join owners groups for each on FaceBook, and ask the owners what they think. You'll get lot's of negative comments on every brand so be prepared for that. Any time you sell thousands of RV's you're going to have some haters. But I think you'll find that the Grand Design owners will also have lots and lots of positive to say.