I'm sure others with more "experience" will chime in, but our experience was...
We looked on line for an upper-grade class "B+". No dealers in our area, so we studied the on-line info for each manufacturer. Looked at the following..
Forest River Lexington
Jayco Melbourne
Phoenix Cruiser
Winnebago Aspect
Coachman Concord
Itasca Cambria
Nexus
Picked out a size and floorplan that we wanted (almost all will have the same basic floorplan in the same size), and started looking at dealers and the RV Trader to get an idea of what was out there and prices. We wanted a one - two year old unit to save the beating on deperication.
Then I looked up "owners forums" for each of the brand's. Each have their own following. Found that each has it's good and bad as far as quality, even in the mid and upper model's. Found that each had it's "horror stories" as far as missing screw's/nuts/bolts, improper electrical connections, improperly made water connections, ect ect. Found this within all the brands. Mind you, not all units were like this, but it only takes one out twenty to give it a bad rep.
As for what makes an "upper level" different...its the upgrades done at the factory, quality of the materials used (like going from sheet flooring to wood, plastic sinks to stainless steel, off brand tv's to name brands, ect).
A lot depends on what you cand do/fix/repair yourself. The chassies can be taken care of by your local Ford/Chevy dealer. But the "home" is probably going to be your problem unless you live around the corner from your RV dealer. Nearest on to me is 90 miles away, so I fix/modify on my own. I'm new to motorhomes after trailering for many years, and a forum like this one is invaluable. Lot of tips/ help from people who've been RV'ing for a long time.
Just try to narrow down what your looking for, set a year range, and study study study what's available. There are some really good deals out there if you look. We found a Forest River 283ts, 2012, with only 1510 miles on it, used one time. Owners didn't like it. We were the first to answer his listing and got it for a super low price, much less than any dealer was asking for a similar unit.
When you find something you like, go over it like your buying a house. Go over it top-bottom and ask a lot of questions from the owner/dealer. Most private owners know a lot more about their units than the dealers sales people do. Sales people know a little about a lot of brands/styles/models/ their trying to sell. A private owner knows his one n only rv.
Grumpy