Hi, I have a 1991 Ford f150 4x4 and I am wondering what size camper to look for? The truck weighs 5550 with family inside and GVWR was around 6500, when I did the math it only left about 700lbs. So is the max I can go? Or is there something I can do to truck to cary more, airbags? Suspension? Ect... Thanks

Well, that is about all that Ford recommends for your truck. Anything over that figure usually results in extra wear on drive train and suspension components.
Many folks will add air bags or adjustable shocks or sway bars but all that does is make the load stabler. It does not change the numbers.
I am over weight with my Lance 1130. And I have a F350 dually with sway bars, Timbren overloads, Rancho shocks. Those things just make the load easier to carry. It does not change the fact that I am over weight.
So what next? Consider a pop up trailer or a smaller popup truck camper.
Chet
p.s. Welcome to the site!

700 lbs doesn't allow for much of a camper
especially since you'll also want to carry camping gear/clothes/food/etc. Cybervet pretty much summed it up - you can add accessories to make the truck sit level and sway less, but you'll still put a lot of strain on the rest of the truck. F150's often come with passenger car tires, so you would need a good LT (light truck) rated tire also. My biggest worry would be the rear axle, which isn't designed for lots of weight and will cause the wheel to fall off if it fails. On 3/4ton or higher trucks, they have full floating axle which by design will keep the wheel attached even if the axle breaks.

For peace of mind, I'd highly recommend getting at least a F250 to haul any camper. And if you want a monster like Cybervet or I haul around, then get at least a F350 dually or even a F450.