We've never used a travel agent for these trips and we've done quite a few. Once you have your flights, decide on dates for Yosemite and Yellowstone and book those, they get booked up very early. Yellowstone is quite a way north from your route, maybe look to fly in or out of Salt Lake City and use another airport for the return.
Personally, I would fly into San Francisco, then Napa Valley, Yosemite, Las Vegas, Grand Canyon and fly out of Las Vegas. This would miss Yellowstone but in 18 days it would be a tight schedule, and it leaves something to look forward to returning for.
The Rand McNally website allows you to plan a trip with mileage etc, we use it all of the time.
Enjoy yourselves, if you need a bag carrier.........!