Thor Industries is the largest maker of RVs in the world, and they own several brands besides Thor itself that they've acquired over the years. Some of their model lines are definitely low-end and not of the best quality, but they also make some that are towards the upper end such as Airstream trailers. One reason you hear more stories about problematic Thor's than (for instance) problematic Winnebagos is that there are many times more Thors, both good and bad.
Thor Industries has about half the RV market. Forest River has about a third. Everyone else has a piece of the remaining 15 or so percent of the market. (That's including together all RVs, trailers and motorhomes).
In terms of rentals, at least Cruise America and I suspect some of the other larger rental companies contracts to have the units built to their own specification. They maintain them pretty well, and do a pretty thorough job of refurbishment before selling them. For what it's worth, when buying a used RV, I would much rather buy a lower-tier brand unit that has been well maintained and is in good condition than a higher end brand that has been neglected. Condition trumps brand pretty much every time for me. The only possible exception might be if I was specifically buying a project to restore, maybe a dilapidated GMC motorhome.