As said it should not over rev due to automated shifting. I wouldn't run it over about 5K on extreme engine braking downhill. Typical RV use is 5K miles per year. Unlikely engine is limiting factor on RV life no matter if it is seeing high revs from time to time.
If you really want to pamper the engine, adjust speed/gear to 3700-3800 limit on grades. Seems to be a good pulling speed on hills.