Your truck is going to be the limiting factor in a fifth wheel purchase decision because of rear axle and rear tire carrying capacity. If you want to be safe and keep the load within weight limits I would advise you to load the truck with fuel, kids, and camping gear. Pretend you are leaving for that vacation trip and get a scale weight of each axle and gross weight.
The actual ready to travel weights and the door tag in your truck will tell you how much carrying capacity is left. Pay particular attention to the rear axle weight and capacity.
Assume any trailer you look at is going to weigh in at maximum gross vehicle weight which is printed on the VIN/Weight sticker on the left front corner somewhere. That is realistic with your family size, could even be a little light.
Then calculate 20% to 25% of the trailer's gross vehicle weight. That is approximately what the trailer's kingpin will weigh, ready to travel on the trip you mentioned and that is the weight that will ride on the rear axle of your truck. If it exceeds the carrying capacity you've already calculated, pass on that trailer. It is too heavy.
It will be a mistake you will seriously regret to overlook the trailer's GVWR tag or assume it can be towed on vacation trips at its empty weight or what the seller tells you it weighs.
Also, don't believe the defensive myth that trucks are underrated by their manufacturers on the conservative side and can actually carry more weight. That is self-deception and will get you in deep trouble. Your rear tires probably have a capacity of about 3,042 lbs. per tire for a total rear axle capacity of 6,000 lbs.