We like to drive less than 300 miles in a day, but we usually don't have a schedule. Some days, you just can't help driving more. I will note though, that some 400 mile days are much, much longer than other 400 mile days. So, we now consider time rather than distance as our rule.
400 miles from New York to Virginia is a lot longer and more tiring than 400 miles from Virginia to South Carolina. 400 miles with a 30+ MPH crosswind across Texas is a lot longer and definitely more tiring than 400 miles across Texas with a 30+ mph tailwind. 400 miles on 101 up the west coast is a lot longer than 400 miles on 90 in Montana.
So, if we are planning ahead for stops, we look more at terrain, road type, typical wind conditions, traffic conditions, and usual construction than we do how far it is. We can cover 400 miles in less than 7 hours on a super highway in the center of the country (we did it a couple of weeks ago). On the east coast, between New Jersey and Massachusetts, depending on the above, we might cover 250 miles in 7 hours.
Another aspect we have discovered - the longer we have 'rested' (stayed in one place) the more likely we will be able to drive longer. If we have been somewhere at least 4 days, 300+ miles is no problem. If we drove 300+ miles the day before, we are dragging about 50 miles before we get where we are going. This is especially true if we have had to drive through an insane gauntlet of city highway, such as around Atlanta or Indianapolis or Jacksonville.
We also have noticed that if we actually stop, get out for a while and wander around, longer drives are easier. The days where we just drive with quick pit stops seem to drag forever.
So, consider time as your limiter rather than distance. Start with the supposition that you will cover 50 miles per hour (this allows for pit stops). If parts of the trip are in a congested area or on secondary roads, add a half hour for each 100 miles. If it is in a mountainous area, one of you has the bladder of a gerbil, or the area is notorious for crosswinds, add 15 minutes per 100 miles. You'll come out closer to the 'mental' drive time.
Your partner is right about 400 miles in car vs. in an RV. Driving an RV requires a lot more attention and patience from the driver. The passenger, not so much unless they are a passenger seat driver.
Good luck and enjoy your trip!