Attention Seasoned Trippers!! Needed: Route advice SwFl to Seattle.

Looking for advice on the best route from SW FL to Seattle. My family and I are convoying with a F-150 hauling a motorcycle in the bed and trailering the other bike. We will also have a 27' UHaul towing my Honda Element. We are looking to the best route thatcan provide the best fuel economy and least trying on our vehicles. It was suggested to stay south and come up through CA. OR and through to Seattle. Our original thoughts were to go on through to Chyenne and then either cut over to Salt Lake to Boisie through Walla Walla, Yakamia and over Snoqulami. Or would it be best to go further north before cutting West which takes us through Montana. We want to keep interstate all the way.
Please, if any seasoned trippers can provide some insight-your expierence is most appreciated! Thank you!

Welcome!

Your least taxing route on the vehicles will be:

I-75/I-24/I-64/I-70/I-29/I-90.

Montana concerns me. Are we looking at several steep grades?

All interstates are designed to be as gradual as possible, and there really are no steep grades anywhere on the interstate system - even crossing the heart of the rockies on I-70 is only a 7% grade.

But Montana actually has the lowest Interstate crossing of the Rockies anywhere north of I-10, reaching its highest point at just over 6,000 feet. By comparison, I-80 actually gets up to nearly 9,000 feet in Wyoming.