There's 3 weights for that 5th. The UVW, pin and axle. The UVW is the total of the axle and pin. If you hitch up and hit the scales and when you roll across them you'll get the trucks rear axle weight and the 5er axle weight. The 5er axle weight won't include the pin weight, that will be included in the trucks rear axle weight.
If you load that 7000lb UVW 5er up with 1000lbs of gear, then the loaded weight will be 8000lbs. The pin will be apr @20% 1600lbs. Now drive to the scales and when you cross them the rear of the truck will weigh 1600lbs more than when empty. And the weight on the 5er axles will be 6400lbs. 8000lbs-1600lbs=6400lbs.