Pretty common problem from another forum. My 2012 Thor had the same problem. The problem is not in the cruise but in the software loaded into the computer. I took my coach to numerous Ford dealers and the response was they couldn't do anything for it,"that's why you have a warranty" was the typical response.
I agree it isn't right, and if continued driving with this issue will in time cause severe damage to the engine. I finally fixed the problem by installing a 5 Star tune map into the computer. I hated to spend the $, but it was worth it. The MH now drives, shifts and downshifts like it should. Stays in a higher gear much longer, without all that extreme downshifting.
Trust me, it is worth the money, if nothing more, than in the peace of mind and not having to constantly shut the cruise off to prevent the extreme high rpms from downshifting to a too low gear.