Take specific note of sound deadening upon entering and compare trailers. Shopping on a rainy day is particularly effective. Goes to insulation quality. Large storage compartments. Can you get to the bathroom without putting the slide out more than a little. Can you get past the fold out bed in the kitchen area while deployed. Ducted heat and air. Extra room in the bathroom (corner baths tend to have more). Access to utilities like water heater, converter, heat, plumbing, electrical (you WILL be looking for these at some point, trust me). Try out the beds for function and placement, they won't be comfy but they should be solidly built. Fit and finish.
And most of all, think about the layout and if it will work for you for more years than you think you'll need it. Kids, guests, etc. Run everything on board, all utilities and doors and drawers...everything. There will be surprises but you can minimize the obvious ones.