I don't look at miles but at hours per day behind the wheel. I try to keep it to 4 hours or less and on average we spend 2-3 hours per day during a 2-3 week trip. 6 hours is the absolute maximum time unless we have to get across a "no man's land" where there is not much in terms of options as to where to stay other than a truck stop parking lot or a Wal-Mart type of big box store and that is something we will never do.
If the distances round trip are going to be more than 4,000 miles we fly to a location where we can rent and RV or if an RV is not available we will rent a car and stay at inns and bed and breakfast places. Add up the fuel costs to drive across the country and back again and compare that to flying and renting and the costs will be nearly the same but you avoid all the time driving along the highway in places you do not particularly care to be. We have rented RV's in Alaska and in Canada and it was without a doubt better than driving there and back in any vehicle.