There are a number of 40 ft diesels out there there exceed 20,000 on the rear axle when loaded for travel, particularly with rear slides. I had a 2000 40ft Monaco Windsor with only a front slide and the rear weighed in at 19,700 when loaded fro travel. Monaco rated the rear axle at 20K, but it actually was a 21K axle.
Some coach manufacturers used a 23k (or 21K) rear axles even though until just last year the Federal legal limit was 20k on any one axle (less in tandem). Some manufacturers used the 23K (or 21k) axle but still used the 20k for their rating sheet.
But before you worry, get it weighed. Preferably all four corners, but if not available at least go to a Truckstop (or gravel yard or other places with truck scales) and get the front and back axle weights. For setting tire pressures do what Gary suggested and assume you have a 5% imbalance left/right. Use the pressure for the higher weight on both sides. Some of us add an extra 5lbs to what the tire charts list for our weight.
If you are over by only a small amount, then review how your coach is loaded and see if you can move heavy items forward.
If you're over by at lot and can't move or eliminate weight, then I would call Gulfstream or the chassis manufacturer and see what the actual rating is for the rear axle on your coach. It may be more than 20K. Also ask them if any other components of the rear suspension limit you to 20K.
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