If you are trying to avoid high elevations for health reasons, then by all means stick to the south in NM and take I-25 north.
However late September in CO means aspens. If you have four days, might I recommend: I-5 to I-17 to US 89 to US 160. You'll go through Durango and Pagosa Springs (scenic drive) then your only major climb is Wolf Creek Pass which is a good climb but not technically challenging. Continue on US 160 to Del Norte and take the CO 112 cutoff to US 285. It's an easy grade over Poncha Pass then continuing north on US 285 to Denver. Trout Creek Pass and Kenosha Pass are no big deal. You'll see lots of aspen and great scenery along this route. Less than 1200 miles this way which means 300 miles per day. If you can get to Durango by the second night (800 miles) you'll have two days to really enjoy the scenery through CO. Many thousands of RV's travel over this route every year.