I just went down the I-5 grapevine 2 weeks ago with my 33' 1994 Holiday Rambler towing a 2005 Dodge Caravan/tow dolly with a motorcycle inside the Caravan. At the top I simply took it out of over-drive, shifted to a lower gear and off I went (YEEHAAA!). Whenever I felt uncomfortable with my speed I'd slow it down 5-10mph with the brakes. I had absolutely no problems. If I did have problems, about 3/4 of the way down there's a "runaway truck" ramp you can drive onto that'll stop you.
Steve
PS Before the trip I swapped out my 20 year old DOT 3 brake fluid for DOT 5.1 (not DOT 5) severe-duty hi-temperature brake fluid. That gave me a lot more confidence starting out.
