There have been a great deal of discussions on this subject on this board- you might try a search. My Fifth wheel is a full standup bedroom = high profile = pulling a brick wall. I have yet to talk to anyone who has seen mileage improvements of more than about 5-7%. However, I've heard that they do tame alot of the drag and jerkyness- in my opinion, that alon is worth it. I'm just waiting for some more OT money before I look into a Taylor. Real expensive, but real nice. There are less expensive fiberglass out there, but they don't look as nice.
Kev