I never use fast idle either and I never have had my coach dragging because of the use of the air brake. That is why I suspected maybe there is some other problem which could be the height valve. It is not uncommon to see them go bad, and the air bags either the front or the rear two separate valve if they get stuck in the exhaust position the air bags will not inflate.
My air pressure will build up pretty fast even when the air has been dumped. It should not take anymore then five minutes.
Some of these posts are funny especially the one's that specify for this person to go and study a commercial test book to learn about air brakes. It does not take much to get used to driving a coach with air brakes versus hydraulic brakes. What I would suggest is to take the coach out in the country and drive those back roads and get the feel of driving a diesel pusher. Once you drive a few miles you would never go back to a gasoline motor home. You would feel more confident driving that big diesel then you would driving your automobile.