We also removed the cabover mattress and use ours for storage. Put down a a roll of anti-slip mat and found bins with a clear door that opens on the side so you can see the contents. You can store a lot up there. We store dry food, extra bedding, camping gear, clothes, etc. It's all easy to get to and there is a curtain that pulls across to cover our "attic". We have a large storage area (105 cubic feet) outside under the bedroom and with the cabover space we are fully packed with lots of storage room to spare even though ours is only 26 feet.
I agree you won't see much of a difference in mileage if you get one that has plenty of headroom. Some of the lower profile "B+" units may get slightly better mileage but you will sacrifice either headroom or outside storage space.