If yours is a gas-powered motorhome, you downshift to get decent engine braking. If it's a relatively new Ford chassis (with the five speed transmission and tow/haul mode, rather than the four speed with the "overdrive off" button), you can put it in tow/haul mode and it will automatically downshift on downgrades when you use the brakes, among other clever transmission programming adjustments.
Don't be afraid of letting the Ford V10 rev at a high speed. It's perfectly capable of doing so without harm, even if it sounds like it's about to shed its parts in all directions. The automatic transmission and engine computer programming won't let you dangerously overrev the engine under any normal circumstances. (I suppose backing down a very very steep hill at high speed you might get it to overspeed in reverse since it has no gear to upshift to...so avoid doing that.)
If it's a diesel without any sort of engine brake, you don't get a whole lot of engine braking by downshifting; that's the nature of diesel engines. (It's because they don't have a throttle plate in the intake, so there's no real restriction on the air pumping through the engine, or so I understand things).
If your coach is rather squirrelly on the road, look up the "cheap handling fix" or "CHF" in these forums (if applicable to your chassis). It's reported to work wonders and costs essentially nothing—just a set of adjustments to the suspension.
