I assume you are talking about at least 8 months from now, this Fall is over.
My advice is to figure out where you want to go, and base the timing on that, or figure out when you want to go, and use that to work out where you are going.
How far north on the northern leg of the loop? Minnesota, North Dakota, Montana to Glacier then through Washington to Seattle? Black Hills, Wyoming, Yellowstone, Southern Idaho, through Oregon to Portland? Kansas or Nebraska to Rocky Mountains NP, Salt Lake, Reno, San Francisco? Tennessee, Arkansas, Oklahoma, New Mexico to Grand Canyon, then southern Utah, Las Vegas and into Los Angeles? All of those have worked for my northern routes, many interesting place to stop on every E-W Interstate or U.S. highway, but each has its own seasons.
Timing is particularly important for the Fall part of the journey, because Fall is a different time, depending on how for north. Same issues for Spring going into Summer, but backwards.
I've been caught in an early winter storm in Montana before Labor Day, marking the beginning of Fall (that one melted, and it warmed up for another month, but taught me Fall comes earlier in the North).
Fall might be mid to late September in the Colorado Rockies, and on the northern Plains. It can be a month later (October) in Kansas and Nebraska, while the northern tier might already be well into winter, and you could run into early winter storms in the and central Rockies. The last time I crossed southern Colorado at the end of September, I had lovely Fall weather but snow was already piled up into 10 foot mounds at Monarch Pass.
If your Atlanta experience has you thinking of Fall as October and November, then your northern route probably should be not much further north than I-40, which will be early Fall across the Plains but some possibility of winter weather in the southern Rockies. That gets you to the Grand Canyon, side trip up to Zion or Monument Valley maybe, then into lower, warmer desert which could still be blazing in summer heat.
It is not just weather, because there will be RV parks and campgrounds closing for the season, which ends earlier the further north you are.
I made an end of September trip across southern Kansas and southern Colorado, to learn that RV parks were starting to close October 1st. One tier of states lower, that is November 1st. Further north, right after Labor Day.
