6'-4" or whatever it is on my Mega cab, 8'-9" long TC, just towed the boat to Idahoo and back last week.
I used a 24" extension this time, can get away with a 12" ext with my boat trailer but can't turn real sharp one direction becasue it's a pickle fork hull and the ladder will hit the boat, turning 90deg.
Wouldn't be near the issue with a v nose boat.
Main thing is look at the vertical overhang of the camper. Ours stays at truck bed level so the hitch extension can be shorter and the trailer tongue can ride closer to the back of the truck without having any issues with break over angles.
If the camper has a bumper or structure that is lower than the truck bed, the extension will need to be longer than a camper with a flat bottom.