A back cushion would probably help a bit. However, the best thing would be to drive short days. Stay at places more than an overnight and then don't sit in the RV. Get out and walk. Plan on the whole summer for the trip and you'll have plenty of time to see everything. Sometimes we just moved 100 miles or so. Don't rush the trip. Take breaks...just walking around the RV a few times helps. (Inspect the tires, etc. while you're doing it)
Don't give up on the trip. Do it! If you find you're really in pain there's nothing wrong with turning around. At least you would have seen something.
Be sure to stop at Liard Hot Springs for a nice long soak. Chena out of Fairbanks has a hot pool, too.
If you're not taking the trip alone, definitely have the other person drive, too. On ours, we alternated each day so the other can experience the scenery and look out for the critters!