There's no magical number but 6" seems to get thrown around as a good minimum from what I've seen. It just depends where you travel to. I'm at about 7.5 and have been on some pretty hairball uneven logging roads and have never hit.
There has been a lot of change in trucks and 5th wheels over the years. Presumably, the 5th wheels have changed trying to catch up with the ever-rising trucks. My newer Ram truck and newer 5th wheel worked out great despite some fears that I wouldn't have enough clearance. The worst scenario seems to be an old 5th with a newer truck.