Newbie question: 5th wheel with no factory prep?

Don't fool yourself. Terms like "dry" are pretty useless. Remember towing a fiver loads approx 20% of its actual weight directly over the rear axle. Lacking actual numbers look at GVWR numbers as your base line. At least then there won't be any surprises. To get a true care carrying capacity load your truck up like your going camping and hit the scales. Keep your calculated pin plus hitch weight under your trucks GVWR and you should be OK.