For a 5th wheel, you're generally going to look at your maximum payload, rather than tongue weight. Tongue weight usually refers to a bumper hitch.

You'll also want to look at your total towing capacity, and max axle weights. You need to abide by the lowest of those ratings. It doesn't matter if your truck is rated for an 11,000 lb towing capacity, if you can't keep the axle weight down below the max rating.

As mentioned above, you're going to have limited use for a 5th wheel in a 1/2 ton truck. There are a few 5th wheel travel trailers that are within the load range of a 1/2 ton, but even those are limited. Most 5th wheel trailers are going to be up in 3/4 ton territory.