gaffer, the DW and I love our 2011 Keystone 250RS which has exactly the same floor plan as shown in your post. To address your concerns:
#1. Agreed, very little outside storage, but it works for us, and may very well not for your needs.
#2. Yes, Keystone and some other manufacturers went with a ventless system...not a good idea IMO. A fan in the ceiling hatch just above our kitchen area works.
#3. Can't testify to the construction, ours seems quality and solid BUT an interesting observation...our 250 is rated at 5,980 dry, this new one is 500 lbs. LESS...less what? Also, ours has a one-piece fiberglass front cap, MUCH preferred as I've read of many problems of leaky/delaminated front "wraps" like this one.
#4. No lower bunk window...yep, it's dark, stuffy and confined. I wouldn't like sleeping there either. Ours is used only as storage, so it doesn't matter to us.
#5. Dump valve location...not a problem, occasionally I have to use a second 10' section to make the 20', both carried in a plastic tube under the rear bumper.
#6. Towed in salt...would NOT even consider it! We towed ours 12 miles home from the dealer new in slushy/salty snow, it infiltrated one of the brake wiring connections causing a real headache with braking problems until we discovered it.
Considering what you've indicated, I'd pass on it. The design has to fit YOU.