I see Pullman is located at the "in between" point where running up to Spokane vs down to I-84 are similar distances. If it were my trip, and particular when towing, I'd run up to I-90 unless local weather up was unfavorable. As mentioned, you're going to want to have the schedule flexibility to sit tight for a day or so following the passage of a snow event, anyway. The advantage provided by I-90 is relative flatness when compared to I-80 from eastern UT all the way to Cheyenne, WY. From the Idaho Panhandle to Billings, MT, you have exactly 4 passes or pseudo-passes. The remainder of I-90 follows either the Clark Fork River, the headwater rivers of the Missouri, or the Yellowstone River, so there is very little in the way of ups and downs. From Billings to east of Rapid City, SD, there are some long grades, but you're at least out of the higher elevation territory. Contrast the terrain from the junction of I-84 & I-80 at Echo Canyon, UT to Cheyenne, WY which involves crossing the Colorado Plateau at elevations between 6,000 and 7,500' and a single 8,640' pass east of Laramie, with some long grades as well, and I-90 looks pretty good. There are more cities and towns along I-90, as well, so if services are needed, you're in somewhat better position there, as well.

There are no really great options, but the section of I-68 from Morgantown, WV to at least as far as Cumberland, MD was tough sledding for us in early August of this year. There are many fairly short but steep grades. I suspect many of the grades are at or close to the limit on grade % for Interstate highways. We towed a camper trailer through there and it was an unexpected challenge at the end of a too-long day. That said, I-76 (the PA Turnpike) through Somerset to Breezewood is no day at the beach, either, and I am unable to say from the perspective of recent travel which is more diffucult when towing. One certainty is that the tolls on the PA Turnpike will amount to something.

Foy