Keystone (Thor) actually has much larger market share and many more brands than Forest River. For where I sit, Forest River's responses to consumers seem much more reasonable than Keystone when looking at BBB complaints.
Leo - you hit it on the head. Within a brand there is no real quality difference between entry/mid/upper end within the same brand. Generally they mostly come down the same line and use the same construction techniques. What you're paying for (within a brand) is typically trim level. Base model versus fully loaded - they call that "quality" - but it's not, it's trim level.
If you want to address actually build quality, you need to look at other brands and (if you've got time) actually look at where/how they are made. DRV is a high end 5th wheel brand. It may not fit your budget, but there is a lot more quality there (and typically a lot more in terms of trim level).
Sometimes you can see quality attempts with RVs that aren't running off-brand tires from China. They have upgraded shackles on their suspension, suspension systems with shocks or that are torque-arm based... You're looking for things that don't typically sell to most consumers like cabinet trim and how nice the couch looks.
Assuming you're not actually going high end and need to stay within the area of "most" RV purchase price, my advice is to look at factory warranty and have a look at how the factory responds to consumer complaints. Jayco has a 2-year warranty, they support original and non-original customers direct. Keystone, however, has a 1-year warranty. Although they will respond to an email inquiry, they will simply refer you to a dealer in their "national" dealer network. And the first thing that dealer will ask is "did you buy it here?"
All brands have issues - and I include the high end brands in that too. You can't buy your way out of issues - these are things that are assembled by hand, relatively quickly and they are prone to errors. Expect to spend 3 hours doing your own inspection before you drive off the lot with any RV.
PM me if you want more perspective.