Richard, The purpose of the tag axle is to spread out the weight load on the rear of the coach also with a coach of this length it helps to keep it from porpoising traveling down the road. In the rear engine compartment you should find a small air regulation with a pressure gauge on it to be able to adjust the air pressure load on the tag axle, the more you load the tag the more weight you shift to the front axle. My front end is really heavy so I only run about 6K on the tag, which I think is about 30 lbs on the air pressure gauge, I would have to go out and confirm that.
Not sure how you coach is setup but in my 06 PT there is a rocker switch in the left hand console labeled Tag Axle, when the switch is turn on it will lift the tag axle. If you are backing up and making really tight turns you probably should lift the tax, it puts a extreme amount of side load on the suspension with it in the drive position. On my coach if I am turning in dirt or gravel with the tag lifted it will plow really bad. I try not to make real tight turns while backing up. Also on my coach if I try and drive with the tag lifted and get over about 5 mph the alarm goes off and the tag will drop down to the drive position and the alarm will not stop until the switch is turned off.
Hope this helps a bit.