Absolutely! This will work. And if you have a second person to help turn the key, will work just fine. It has been my experience that the leads on multimeters are not long enough to reach from the battery to the cab. Or from under the hood to the cab of pickups, cars, boats, etc. I've made longer leads to get me by in a pinch, and is possible, but is the readings accurate at that point?
Alternatively, you could spend less than $40 for a little digital load tester from Amazon
(link) that you can use yourself without ever needing to turn the key. The one I linked is the one I use. I keep it with my electrical testing equipment, and if I ever suspect a battery, its simple to grab and use.
Just a thought while we wait to see what happened to this particular tractor.