You can't know the proper pressure setting unless you weigh all 6 positions individually. If the front axle weighs 15,000# that does not mean that each tire is carrying 7,500#. The left front could be 7,000# and the right front 8,000# (let's hope not, though). You have to set tire pressure based upon the highest weigh carried by any tire on that axle.
The tag axle will carry the least amount of weight and it's likely that 85-90# is correct. I can't compare your coach to mine (or any 2 coaches) but I have 110# on the front, 100# on the drive and 90# on the tag.
What I post is my 2 cents and nothing more. Please don't read anything into my post that's not there. If you disagree, that's OK.
Can't we all just get along?