I guess I'd aim for 8 hours or less a day of driving, but might consider a little more if needed to work things out. That means maybe 400-450 miles, I guess. As much as anything, I find the limiting factor is arriving at the destination campsite at a somewhat reasonable time (before dark and before my neighbors are trying to sleep and, preferably, before it's completely too late for supper).
In a car, I aim to keep it below about ten hours; much more than that and my driving skill (and safety) starts to take a rather alarming drop. The motorhome is a little bit more tiring to drive than a car.
I rather enjoy driving, truth be told. I have at times done a little driving over the night with the motorhome, stopping to snooze for a few hours here and there at rest areas or similar places. It's very convenient in that way to, like a snail, carry one's house about on one's back.