contact the Tractor testing lab @ University of Nebraska, they have tested all tractores sold in hte us for decades and have all sorts of info availibel
www.tractortestlab.unl.edu/

SBI
Central Ne.,USA

Well, sounds like what you really need to know is how much power and how much drawbar pull is required for your trailer.

Do you need an estimate or an accurate number? Accurate number is easy, setup a test apparatus to measure drawbar pull and pick the speed which you desire to go up the hill.

power = force x speed with the units changed up accordingly.

What surface are you pulling on?  Driving up a plowed field takes much more power than paved road, the rougher or softer the terrain the more tire mechanics and soil mechanics matter.

DITTO...  we covered this in our Power and Machinery class at OSU.