Even with a Sherline scale, you need to handle the height - a long stick between a scale on the ground and the king pin wouldn't be good. Since you must have a truck if you have a 5.0, I suggest placing the scale (whatever arrangement you choose) in the back of the bed of the pickup, on blocks or whatever is structurally sound and available, to support it at a suitable height. Unfortunately, the truck will move down as the weight is applied to it, which is dicey for the scale setup; I might put jackstands under the truck frame to prevent that.
I stuck a pressure gauge on the port of a small hydraulic body repair kit cylinder
to make a hydraulic scale functionally like a Sherline (although you need to calculate piston area to determine the calibration). A Sherline is more convenient, and has a better base (wider), if you can readily get one and are willing to pay for it.
The accuracy of the lever method (shown from the earlier discussion) depends on knowing the ratio of the lengths accurately, so it would be wise to put something with an edge or other line contact on the horizontal beam so the load is at that line; the illustration uses pipes for this, but I would use a piece of angle iron, vee point up, for stability and ease of measuring positions.
And of course if using the lever method a 2x4 stick of lumber would be entirely inadequate for the pin weight of a 5.0TA... I wouldn't even try that with the tongue weight of a 17'. I wouldn't even stand on the middle of 2X4 laying flat across supports three or four feet apart - why would anyone support a trailer tongue that way, if it's too heavy for a bathroom scale?