I've done 4 for 18 hours with guys all averaging 220lbs and all over 6 ft.
As long as nothing else needs to be in the cab, the a/c works good, and you are all ok with each other it can be done. 4 was not bad at all, 5 wouldn't be great but is doable.
Other thing to consider, and i hate to be morbid. But if an accident happens on the interstate and it's a bad one you are putting a hell of a hurt on your family.
Not that 3 in a vehicle wouldn't, I'm just saying taking 5 from a family in one shot would be beyond earth shattering.
Lot of pros and cons either way.