I WOULD use the tractor. You already own it vs making another purchase. I did this very thing one time during a loss of power at a campground during the winter. I hooked a vw bug to the trailer via jumper cables and idled the car all night.
While the alternator on the tractor is typically a lower output, I would imagine it to be more than adequate for battery maintaining. Personally; I'd run the tractor speed up a little bit, maybe 1500rpm or block part/all the radiator to get done heat on the engine to reduce cylinder washdown.
You're not going to burn enough diesel to equate the cost of a cheap generator.
