Not really. I threw a couple rags in the bottom of my diff to absorb some in the pool there (and prevent it from spilling over onto the sealing surface).
Just drain and refill on time. No need to flush. BTW, does your manual say to go with 75w90? I'd think a 75w-140 would be better for how heavy your motorhome is.
I think the correct answer is, "Most times, no." I have found little pockets of break-in wear metals, clutch lining from LSDs, casting sand, etc, hiding in pockets. No real problems with the diff but this stuff is sorta "gooped" there and could be released by the new oil. That's a case where a little brakleen and a small scraper or brush is in order. Not all diffs have suitable pockets for this stuff to collect, but some do.
