Of the OP's list less than 1/2 of the items are anything close to substantial or surprising on an RV at this price point. Things like loose hinges, gaps at countertops and other finish items are, unfortunately, to be expected in an RV at this price point. Sure, you can hound your dealer or some warranty. If you do and are successful congratulations, you have more patience than I do.
Maybe I'm lucky having the skill to just deal with some of this stuff on my own. A broken landing gear and leaking roof? Yes, those are defects that should be fixed. And it sound as though the OP may have a greater than typical share of problems.... but not unheard of by any means.
If it makes one feel better to completely rule out a manufacturer go for it. Just don't expect anything better from the "lottery" with the next one. These are RVs - not custom homes.
And for the record I've never owned a Keystone product and am not a defender of them. I put them in the same category with other units of the same price point.