Ford's heaviest gas motorhome chassis has a GVWR (total weight limit of coach) of 26,000 pounds and a GCWR (total weight limit and coach and what it is towing) of 30,000 pounds. Depending on your needs a 40 footer might put you in a bind with regards to payload and the vehicle you can pull behind it. Not necessarily but possibly, it's something to be aware of.
Now Forest River is building that 40 footer you like on the 24,000 pound chassis and I'd be leery of that. I'd be leery of the 252" wheelbase too; that's pretty short and will result in a long overhang behind the rear wheels; that means wide ass tail swing and possibly poor straight line stability.