You're asking a lot of great questions.
As far as floor plans go, what size trailer are you considering? You mentioned your looked and many in a size range and "they are all the same" suggests you might have looked and several brands but it's possible they are all under the same corporate umbrella. Forest River as an example includes Cardinal, Salem, Rockwood, Sandpiper, CedarCreek, Blueridge, Wildcat, Sunseeker and several more. Thor Industries has Airstream, Heartland, Keystone and Dutchman as product families and a bunch of brands within each family. In many cases, the same floorplan with changes in lipstick and mascara make one think they are looking at competitors. You'll find regardless of manufacturer, certain sub-components are the same - fridge, furnace, stove, water heater, that sort of stuff. Others have mentioned that certain frame manufacturers command the market which is true but like any tier ll producer, manufacture per the tier l specification or will design and produce for the tier l.
So how do you get answers . . . Factory tours is one way to look beyond marketing hype and owner opinions which are good but mostly based on successful ownership of a single brand. I would suggest a tour of DRV, Jayco, Heartland & Dutchman just to name a few. DRV would be an example of a higher end build with the others being middle of the road to upper middle class.
Also know that the dealer is generally independent from the manufacturer and not always "in the know" when it comes to construction knowledge.