I tow a 40 foot 5th wheel with a 2014 srw diesel f350, your going to hate having a diesel, lots of power, quiet, good fuel milage empty or loaded. the 5th I have is a cougar 330RBK and it will sleep 4 in the back bedroom, two in the main bedroom, two on the pull out cough so thats 8, and I have never looked to see if the table makes down also, that woudl be another 2. the biggest thing is to find out what they say the factory 5th wheel hitch weight is, add 500 lbs for overage and the hitch and look for a truck that has that pay load capacity with everyone loaded up.
you never mentions a spicif rv, so anyones coments are going to be guesses, but there are plenty of lighter 5th wheels out there that will sleep what you want.