If I understand you correctly, you're wanting to find out what the coupling height is when the trailer is level so you can match the drop on your hitch so that the trailer pulls level when attached. Complicating that the trailer is on a slanted driveway.
I assume the trailer is not attached to the truck, so what you could do is to raise or lower the tongue jack until the frame of the trailer is the same height when measured at the front and rear. This will give you the tongue height, then you can deduce the hitch drop requirement to be level. don't forget to take into account any suspension squatting of the truck.