First question, why a 38-40' 5er? Have you even looked at models yet? What type of traveling do you plan to do? Seasonal moving from north to south & staying put for the season or some real traveling around the US & Canada?
For seasonal moving the 38-40 maybe fine for the CG site at either end. For exploring the country it will be too big. Some of the most desirable CGs are the state parks & National parks. There are very few sites accomodating rigs of even 35' in most of those parks. Something to think about.
There are plenty roomy 5ers in the 30-35' range. How much room does one really need? What is needed is smart use of room. If you are planning extended travel you need large galley space. Those little side only galleys just don't cut it. Counter space is needed not floor space.
Also, don't be dazzeled by the glitz. Look for quality of build & hardware used over gizmos like disappearing TVs & automatic leveling.