[Texas TC]

IMHO, and I think someone mentioned this before, I would try air bags before I changed out the springs. Air bags would stabilize the rear and would cost a great deal less than changing the springs. If they do the trick, fine. If not, springs might be the next option. Also IMHO, I am a little concerned about changing to stiffer springs.

Back to my coach, the 34' is so light that I never compress the rear springs to get them soft. Loaded I only am putting 12,400# on the 17,500# capacity which I feel adds to the stiffness in ride at the rear of the coach.
Having said that, your numbers show that you have exceeded the capacity which may be causing the bad ride you are experiencing.

The decision is up to you. If it were me, I would try the air bag route first.
The opinions rendered are mine alone. No GS/International engineering opinions are stated.

Good luck and keep us posted on your decision.