If you go back and read what I wrote several posts ago, I suggested you could do the trip in 2.5 days, and I even included where I thought you should aim to stop.
I've also said that 500 miles a day is a good number to shoot for for a full day on the road in an RV (assuming Interstate driving, not 2 lane scenic byways), so I'm not sure why you're now talking about 400 miles being the max?