Accept that you’re going to be driving slow and other drivers will be expecting you to be driving slow, so just be patient.
Anytime you are driving say 20-30mph below the posted speed limit, turn on your hazard lights to alert other drivers you are going extra slow, as a good safety precaution (if traveling in a state where this is allowed).
Before you even start going up the hill, downshift to a lower gear so you have some extra power.
Put your transmission in Tow Haul Mode (if your vehicle has it) which uses different gear shifting to keep the engine in a more optimal range. We leave our RV in Tow Haul Mode almost all the time.
When preparing to go down the mountain pass, bring your speed way down before you even begin your descent and stay in Tow Haul Mode. This allows you to use more engine braking and gives you room to increase your speed safely.
If the RV doesn’t automatically downshift itself when going downhill, firmly press the brake to force the transmission to downshift. This will increase your engine speed (rpm).