In addition to what others have said, another thing to keep in mind is that most manufacturers assemble their trailers using components purchased from just a few suppliers like Suburban, Atwood, Dometic, Dexter, Lippert, etc. So while there is some variation in build quality a lot of the critical components are the same.
Take the complaints you hear on the internet forums with a grain of salt. People with problems tend to vent while those that are satisfied don't say much at all. So you will find way more complaints about a particular manufacturer that you will praises.
The other huge difference maker in my opinion is the dealer. A good dealer evaluates the rig and takes care of the minor issues before you even pick it up. They will spend time with you going through the proper operation of all of the systems so you are comfortable with how things work. They take care of you if you do have problems after the fact. I think a lot complaints about a particular brand are really due a dealer who is only interested in moving their inventory as quickly as possible and could care less about the customer once the trailer is off the lot.
Good luck!