The OP may well have the 4R100 transmission like we do. Many of those who offer pat solutions have not experienced the totally absurd behavior of the V10/4R100 powertrain. I've driven the newer (5R110/TorqShift) like I think TPI has and found its behavior a little better.
My belief is that the Cruise Control is way too "strict" with no adjustment possible. Five-Star's website suggests that it can ease the problem. I have not called them or tried their system. The RV can be driven off cruise to reduce and nearly eliminate the nonsensiscal downshifts, so I suspect adjustments to the PCM could also reduce them.