We drove the 90-82-84 stretch a year ago, also 40ft DP towing. All good road, you won't have any problems, but there are a few grades in eastern OR, and again a succession of grades in southern ID and northern UT, lengthy rather than steep (iirc), leading into Salt Lake. My Cat started heating, but that was my own cooling problem, rectified in SLC. Running normal you shouldn't have difficulty.
There's no shortage of RV parks and campgrounds along the route, as well as things to see. We stayed at a KOA in Pendleton, (Pendleton Mills Outlet), which was a nice place, and I would also recommend the Mountain Home RV Park in Mt Home, ID. Very nice folks, park is very neat and well kept.
Do not miss the Oregon Trail Interpretive Center, near Baker City. One of the most fascinating stops we've encountered. Also, a hidden gem, the Shoshone Falls Park, along the Snake River at Twin Falls, ID. Spectacular. Not to mention an absolute host of grand vistas along the way.
I'm not sure if I'm misreading you, but if you're concerned with grades you might want to consider I-80 rather than I-70. If you take I-84 from Ogden to connect with I-80, you can bypass the long, steep upgrade on I-80 through Parley's Canyon immediately east of Salt Lake, and after that the highest point through WY is about 8500.
I-70, besides being more distant to get to, also has those two looong high altitude grades up to Vail pass at about 10000, and the Eisenhower Tunnel at about 11300. Granted, I-70 through the CO Rockies is probably the most scenic highway in the US.
Your choice. Enjoy!
John & Diane, Fulltimers. RVM103 NHSO
On the road since June '12 with Lincoln, the guard cat.
2002 Dutch Star 40, Freightliner, Cat 3126, 2004 Element