Try Stevens Pass near Skykomish, WA. , La Manga and Cumbres Pass on RTE-17 in CO. (went over that one by accident in my big truck... . OOPS!)
The hardest "steady" pulls are Cabbage Patch, Pendleton OR. , grades North and South of Camp Verde, AZ. (I-17), Loveland and Vail Pass, CO. (eastbound), Homestake Pass, Butte, MT. , Dunsmuir, Tehachapi, Cajon, Grapevine Passes, CA. ,
Anyone been down the RTE-140 grade between NV. state line and Adel, OR. ? That is the one grade I do not like going down with just bumper-pull, let alone in big rig. No guard rail with sheer cliff and S-turns all the way down. 8-9% grade. Kinda reminded me of The Death Road in S. America.
Below is my old 359 Pete atop Crater Lake Pass, OR in 8' of snow.