My previous RV was a Sprinter based Itasca Reyo. I drove it several times on I70 up and over the Rockies. I didn't really have any problems with it.
When I was towing the Jeep Wrangler 4-door, I was able to keep my speed up to about 45 going up the mountains. Now, if I got caught behind a slower moving semi, I didn't have enough power to ever get back up to 45.
Now, when I wasn't towing the Jeep, I was able to keep a good speed and would only occasionally slow down to 55. If I got stuck behind a semi, I would have enough power to get back up to speed (eventually
).
Going down wasn't a problem either. Simply downshift to 4th or 3rd to keep your speed down.