Doug, there were two arrangement for the tag axle switching. On the earlier models, typically, there is a two position toggle switch on the dash near the leveling switches. In the down or normal position, air is sent to the tag axle so it shares the load on the drive axle. The air pressure is adjustable by a valve near a pressure gauge in the engine compartment. As you increase the pressure on the tag axle, the tag removes weight from the drive axle and increases the weight on the tag with a smaller increase in weight on the steering axle. As Tech Talk reported, typically 25 psi is a good setting for the tag. However, you can not determine the correct tag axle pressure without weighing your rig on each axle. In the up position a warning bell goes off and the pressure in the tag slowly goes to 0. This is for low speed tight turns or situations where you might be on wet grass and the drive axles are slipping. This switch MUST be returned to the normal position for driving at speeds above 15-20 miles per hour as the rear tires and axle are not designed to carry the entire weight of the rear of the coach as speed increases.
On most of the newer coaches, the two position switch is replaced by a three position switch. This switch has a normal position in the center. Pressing the switch into the up or down position with either dump the tag for us as stated above or increase the pressure on the tag to take additional load off the drive axle temporarily. This was done to allow coaches that normally had more than 20,000 pounds on the drive axle to reduce that load below 20,000 pounds as they crossed the scales on certain toll roads that have inline scales and will not allow you on the toll road if any axle is over 20,000 pounds. A fully loaded early 90's Newell can have 22-23,000 pounds on the drive axle as the drive axle is rated for that load. By activating the tag axle pressure increase switch, several thousand pounds is transferred to the tag axle from the drive axle and several hundred pounds are transferred to the steer axle. Use of this setting on the tag axle switch for extended periods could result in the tag axle being overloaded.
When I bought the coach, someone had set the pressure on the tag to 120 (obviously didn't know what they were doing), the weight on the tag axle (empty coach) was 15,220 pounds while the drive axle was only carrying 10,540 pounds. After loading the coach with about 3,700 pounds of 'stuff' and reducing the tag pressure to 30 psi, the tag was down to 6,480 pounds and the drive was up to 22,020 pounds. I checked by tag axle pressure versus weight in Kerrville, Texas a couple of years ago. There is a gentleman named Gill that will come to your site with portable scales. You drive up on the scales and it gives you readings by wheel position (right tag, left tag, right drive, left drive, right steer, left steer). I changed the pressure from 20 psi to 55 psi in several stages. The weight on the tag axle increased by 3,620 pounds. I need to run at 40 psi to keep my drive axle under 20,000 pounds. This keeps the tag axle still under it's rated 10,000 pounds but does add about 330 pounds to the steer axle also.
It sounds as if you have the two position switch which I also have in my 1992. The tag doesn't have springs on it to physically lift it off the ground like say a Prevost but as you pointed out, with the air dumped, the tag tires are "unloaded". After the air is released (slowly) from the tag, you will find that you can turn corners noticeably tighter (designed for use in tight campgrounds) than with the tag inflated. You effectively reduce your wheelbase by over 2 feet when you dump the tag.
When you are leveling the coach where the rear must be dropped to level, you should also dump the rear tag pressure. Otherwise, there will a significant load placed on the tag axle while you are parked which is not good for the tag tires as they will likely be overloaded.
Just remember to ALWAYS return the tag to the normal position when traveling. The dump (or dump and temporary increase positions with the three position switches) are meant to be used on limited occasions only.
If you keep the solenoids in good condition (or carry a spare and know how to change them) you should not be afraid to use the tag dump. Be aware that those solenoids are about $90 each and you have a tag pressurized and a tag dump solenoid.
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