Noon timeframe youll find I66E not too bad, westbound will start to grow/crowdy by 2pm, stay in the center lane once you get past the Fairfax county parkway until you see the signs for I495 to avoid the various merge points. I495 north from I66 will be probably by heavy, but should not start backing up yet. If you do not get to I495 before 2pm youll find the backup from the American Leagion Bridge all the way through what is called the I270 spur. That wont let up until Chevy Chase or possibly as far as Silver Spring.
I used to drive that on a regular basis when I lived in MD and worked in Fairfax (until I changed jobs and moved to a commuter train.) I still drive it to see family and that traffic pattern still exists even as of last weekend.
I will say one thing, just be prepared. Most drivers could care less if you are a car, RV, or tractor trailer. They drive in their own little world and the me-first self important attitude is definitely the normal driving style around here.