Back when my wife was still working and I had just retired, I took a few long trips (Atl to Vegas,etc) by myself with 2 dogs. I planned on 350mi per day but ended up doing 450 to 500 easy, and that's stopping for the dogs to go the bathroom a lot. Now that the DW is retired 350 is pushing it. Wait for her to get ready in the morning, pick everything up,etc,etc.
After reading many post on here in other forums most people seem to agree that 300 to 350mi is a good day. I would plan on that and keep going if your comfortable. I do try to get into any campground or tight area prior to dark though to many things you can not see ump out and scar/crunch a big rig