The number of passengers you can legally have in a traveling RV is limited by the number of seat belts and restraints in the RV. Each adult is supposed to be buckled in when the motorhome is being driven. Children are required to be buckled in the appropriate car seat or booster seat when traveling.
As for travel trailers, I don't think it is legal to have any people riding in the trailer while it is being pulled down the road.
I think regulations concerning slide-in pickup campers may vary from state to state.
